Demystifying Network.pdf

 INDEX S. No Topic Page No. Week 0 1 Introduction 1 2 Analogy for Week 1 CEO's Problem...

 INDEX S. No Topic Page No. Week 0 1 Introduction 1 2 Analogy for Week 1 CEO's Problem 3 Week 1 3 Discussing the CEO's Problem 10 4 From the CEO's Company to Layers in a Network 12 5 Layers in Detail 13 6 Layered Nature of a Network 19 7 Introduction to Internet Data Capturing using Wireshark 26 8 Network data captured while requesting a website 28 9 What is Cisco Packet Tracer 34 10 Modes of Cisco Packet Tracer 36 11 Getting Cisco Packet Tracer 38 12 Logical and Physical Typologies in Cisco Packet Tracer 44 13 Devices on Cisco Packet Tracer 48 14 Introduction to the Cisco Packet Tracer Activity for Week 1 53 15 Introduction to the campus network on Cisco Packet Tracer 54 16 Loading the page in Simulation Mode 58 17 Inspecting the packets in Simulation Mode 62 18 Editing the dummy website on Cisco Packet Tracer 66 19 Summary of the Cisco Packet Tracer Activity 70 20 Introduction to Anupam's Adventure 71 21 Anupam's adventure brings us to IP Addressing 72 22 Addressing at various layers 74 23 IP Addresses 77 24 Address Translation 79 25 Introduction to IP Addressing 83 26 Creating a network with Sub-net mask 88 27 Nomenclature of a sub-net mask 91 28 Network addresses and Private networks 94 29 Introduction to the Addressing Topology 98 30 Addressing a local network and DHCP 105 31 Addressing a local network manually 109 32 Addressing in Pubilc and Private Networks 113 33 Verifying Connectivity using Ping 118 34 Using network address translation to communicate on internet 123 35 Using Sub nets and Summary of addressing 130 36 Summary of the week 134 37 Analogy for the week 2 136 Week 2 38 Discussion on dabbawala analogy 145 39 From dabbawalas to routers and switches 149 40 What is routing ? 151 41 Static routing in a router in CPT 156 42 How does a switch forwards packets CPT 160 43 How to add static route in a router? (CPT) 162 44 Traveler's dilemma 165 45 Duscussing the Traveler's dilemma 166 46 From Traveler's dilemma to Dynamic Routing 167 47 Dynamic Routing with Distance Vector 170 48 Distance Vector Routing in Detail 177 49 Dynamic Routing with Link State 183 50 Setting up dynamic routing in Packet Tracer 189 51 Summary of the week 192 52 Introduction to analogy for week 3 196 53 Analogy for week 3 197 54 Questions on analogy for week 3 211 Week 3 55 Understanding the new order requirements 212 56 Introduction to Tranport Layer 215 57 Introduction to TCP 218 58 Introduction to UDP 223 59 Exploring UDP on Cisco Packet Tracer 227 60 TCP Connection Establishment 231 61 TCP Connection Closure 237 62 Summay of TCP and UDP on Cisco Packet Tracer 240 63 The story of the delivery fiasco 241 64 From delivery fisaco to Port Numbers 242 65 Application Layer in depth 245 66 Port number in Wireshark 249 67 Summary of port number and PAT 258 68 Summary of the entire TCP IP stack 259 69 Introducing the analogy for week 4 266 70 The secret box 267 71 Questions on analogy for week 4 271 Week 4 72 Secret of the secret box 272 73 From secret box to encryption 275 74 Introduction to security and CIA 277 75 Information Security and Defence in Depth 281 76 Information Classification and Access Control 283 77 Process Management 286 78 Introduction to Network Security 289 79 Network Breach and Countermeasures 293 80 Internet Security 296 81 Securing the Internet Usage 298 82 Internet Security Products 300 83 Personal Computing Device Recommendations 303 84 Responsible Behavior on the Internet 305 85 Best practices for home Network and Media Devices 308 86 Closing thoughts on security 311 87 The story of a family trip 312 88 The troubleshooting approach 313 89 Troubleshooting Physical and Data Link Layers 315 90 Troubleshooting Network Layer 316 91 Troubleshooting Transport and Application Layers 320 92 Troubleshooting Summary 324 93 Troubleshooting Heuristics 325 94 Troubleshooting Challenge 1 326 95 Trouble shooting challenge 2 335 96 Trouble Shooting Challenge 3 350 97 Thats How we Troubleshoot 363 98 Week Summary 365 99 Course Closure 367 100 Course Credits 370  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 01 Introduction I am Shridhar Iyer Professor in the Department of Computer Science Engineering as well as the Interdisciplinary Program in Education Technology at IIT Bombay. This course on Demystifying Networks is meant for students who do not have a core CS background. Everybody has grappled with questions like how does some technology work and so on. And usually when we look up the technology, there is a lot of jargon that comes which hinders the understanding of the technology at a conceptual level. So, this course is meant to provide a conceptual understanding of some key concepts in networking. Hence the course will not going to excruciating details of any specific technology nor will the course remain at a very broad level. One other thing that we are trying to do through this course is to relate the concepts to examples that we see in everyday life. Hence there are analogies, there are puzzles and that’s the format in which we are trying to create this course. The topics in this course will involve some of the standard topics in networking such as layering, addressing, routing as well as some concepts from security, troubleshooting. Essentially, what we will try to do is we will try to look at a network from an end to end perspective, such as a client connecting to a web server and we will try to go through all the layers and what happens at each layer to understand the key concepts involved in the process of sending a request to a server and getting the response. The format of the course will be that first we will start with posing a problem, it might be in the form of an animation or it might be in the form of a text that you have to read and then you discuss this amongst your peers. So, there will be a focus question which each of you is expected to answer in the discussion forum and discuss with each other. Then the week starts where we summarise this problem and identify what are the key takeaways from discussing this problem. After we have done that this entire thing has happened in a real life scenario, after we have done that then what we do is move to the corresponding technical concept, we will try to relate it to the corresponding concept in networks, where a similar ideas are being used and try to learn it in the technical sense. After that of course, you will have to 1 do some practice. So, there is some activity that you will have to do, maybe in the form of multiple choice questions or maybe you may have to download a packet tracer and work with some files offline. All of these we will provide you with instructions. So, you need not worry about it at this point. Once that is done we will repeat this for another topic. So, in each week we will largely try to keep it so that, we do a discussion of two topics, the technical concepts for two topics and the corresponding practice for two topics. At the end of it, we will summarise the week for you and lead into an assignment. Of course, there is no getting away from the assignments. This course will not run in a typical format of showing of slides and having instructors talking head videos. In order to make it engaging for you, what we have attempted is to create the course in a conversational style. So, there will be dialogues which will actually drive the learning of the concepts. The dialogues will be sometimes with me and my PhD students, who are the teaching assistants for this course, sometimes it may be just the teaching assistants conversing with each other and sometimes it may be screen captures and voice overs. In blending all these different formats together we have tried to make sure that the course is interesting for you at all times. Now, let me introduce the teaching assistants for this course. Hello, I am Kavya Alse. I am a PhD student in the Interdisciplinary program in Educational Technology, IIT Bombay. Hello, my name is Ashutosh Raina and I am also a PhD student in the Interdisciplinary program in Educational Technology, IIT Bombay. Welcome to this course on Demystifying networks, we hope you enjoy the journey along with us. 2  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 02 Analogy for Week 1 The CEO’s Challenge If you remember, my TAs had asked a question of how does a website load or how does packets are transferred from the web server to the web browser? So, in order to understand this, let us first take a real life problem. Let us take the problem of transporting goods from one part of the country to another part of the country. In order to help you understand this here is an animation that you can watch. (Refer S lide Time: 00:48) Trekking is a fun and adventurous activity for most, but it can be intimidating to go on your own. No need to worry though. The Adventure Travels Private Limited is organising a hands on trekking workshop in Manali. However, the supplies will be provided by Sports Equipment Inc. Meet Sania, she is the CEO of the sports equipment company. Sania wants to ensure that the equipment reaches on time. 3 (Refer Slide Time: 01:19) It’s important for her business. She hands over this responsibility to her business manager, Amir. Being a responsible manager, Amir immediately calls up the workshop manager at Adventure travels, Meera, to understand their requirements. (Refer Slide Time: 01:38) 4 (Refer Slide Time: 01:42) The workshop is in 15 days. The equipment has to be transported from the company’s office at Hyderabad to the workshop site in Manali. Amir states the delivery requirements we got from Meera to the delivery team. (Refer Slide Time: 01:56) 5 (Refer Slide Time: 01:59) The shipment is over 200 kgs, out of which, 50 kg needs to reach within 5 days. To make sure everything reaches properly and on time, the delivery team has people on contract to deliver the shipment in person. (Refer Slide Time: 02:18) 6 (Refer Slide Time: 02:21) However, there are some challenges. First, Hyderabad has access to all 3 modes of transport however, Manali does not. Not air, not rail only road. Moreover, some equipment cannot be taken by air due to the airline restrictions. (Refer Slide Time: 02:40) 7 (Refer Slide Time: 02:46) The nearest airports to Manali are in Chandigarh and Kullu. The nearest train station is in Kangra. (Refer Slide Time: 02:49) Flights from Hyderabad can land at Chandigarh, from where one can take the route to Manali. Even Kullu is a possibility. However, the flight schedules are quite unreliable. The delivery team takes these decisions and puts the code on each box in the shipment which is used by the delivery boys to reach the destination. The delivery boys head out on different routes with the intent of reaching the same destination. However, some of them might go missing, their phones 8 might be unapproachable, they might have left the job or even sometimes might have misplaced the parcel. Note that it is required that the delivery boys only use public transport for the journey. So, this is the situation. Now it’s time for some questions. Now that you have seen the animation, you are now familiar with the problem. Problem is to transport goods which can be dismantled from one part of the country to another part of the country. The question is how is this transportation to be achieved, given the constraints that were shown in the animation? So, what you need to do now is to answer two questions. One, come up with your own scenario of how you would solve this problem given this constraints. Once you have thought about the answer post your answer in the discussion forum. That is the first part. Second part is to read somebody else’s answer and identify what might be the challenges that persons answer may give rise to. For example, in some cases it may not be very efficient in terms of time, in some other cases it may not be meeting some constraints. So, the second part is you have to comment on another learner’s answers in the discussion forum. Having done that, in the next week we will come back to this animation, we will discuss some sample solutions and the pros and cons of each solution. Then we will relate this to the corresponding concepts in networking. 9  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 03 Discussing the CEO’s Problem Welcome to the week 1 of the course. In the previous week, we saw that Amir who owns sports equipment company received the order from Manali to ship some sports equipment to Manali. Now this order was first process by the billing department which took care of the payment then the order was processed by the dispatch and packaging department. So, the key takeaway here is that the billing department knows its billing process and their dispatch department is aware of packaging and sending the order. So, every layer of the company knows its own function and they send the order to the next layer as they are done with the order. At the end of previous week we had also asked you a question to think about and give your answers in the discussion forum. Based on that we have a reflection question for you what we want is to go through the question, think about it, pause the video, write your answers and then continue with the video. (Refer Slide Time: 01:18) Which of the following are valid solutions to the challenge of “the shipment is heavy for a single person” in delivering the materials correctly on time? Option a - As all shipments are modular, they can be taken in parts by the same person on multiple trips. Option b - As all 10 shipments are modular, they can be taken in parts in multiple people in individual trips. Pause here for a moment, think about both the options and pick one which you think is correct and then continue with the video. So, some of you might have picked option a. While option a is a valid option, it is not efficient because one individual has to travel multiple times, which is time consuming and not practical. I agree and if you look at option b, this seems to be a much more efficient option as the shipment will be sent in time. As all the people are turning at the same time and some parts of the shipment could be sent via cheaper transport mediums. So, it could be efficient on the cost aspect. So, continuing in with the same examples, there were a lot of other challenges too like, every delivery person could carry only 30 kgs of goods with them. Hence, I will discuss in the forum, the entire order of the huge weight had to be broken down into orders of 30 kgs. Also, if we number those smaller modules while they are been broken down, then it will help in keeping track at the other end also. It is easier for reassembling. Exactly. And that would also help if one of the orders got misplaced, they could track that order back or send a replacement for that order. Right. 11  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 04 From the CEO’s Company to Layers in a Network As you just saw, a manager is required to keep track of various things. The manager ensures that the entire package is created and now this package sometimes may need to be dismantled in order to be shift and so on. While this may sound like an artificial example, what actually happens in a network is something very similar. The application layer is what corresponds to the manager in this case. It’s the application layer task to ensure that the request is sent. For example, if we are considering a client and a server or imagine that you are making a request for a webpage, it’s the application layer which ensures that the entire web page is sent from the web server to your computer. Now, this could be a very large page and the same problems as we saw in the analogy may exist in the network. Hence, the page has to be split into chunks. This is similar to what we saw in the analogy as the manager having the dispatch section, where there are different people who are able to carry only 30 kilos, sending things by different routes and so on and so forth. So, at a very rough level if we look at what are the key ideas that have been discussed through this analogy, the first idea is one of layering. So, there are different layers which have different functions. The managers layer function is to keep track of the entire shipment, the next layers function is to make sure that there are packages created and the third layers function which is the dispatch, it’s function is to actually carry the package from one place to the other. Similarly, in a network there are multiple layers we will come to see how that works in a network. The other idea that we have seen, is one of chunking or segmentation as it is called in networking parallel. 12  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 05 Layers in Detail Now, when we try to study this topic a little more formally, then you will find in textbooks that there is something called OSI stack which is mentioned as well as there is something called the TCP/IP stack which is mentioned. (Refer Slide Time: 00:17) So, these are nothing but the same idea of layering that we have seen of 3 or 4 different layers carrying out their functions and talking to the peer layer on the opposite side using protocols. Now, I will not going to the details of each of the separate stacks and so on, because that you can get by looking at textbooks as well as there are other courses which deal with that. So, here I will restrict myself to talking about it at a more conceptual level. Conceptually, let us look at it from the bottom moving upwards. The bottom most layer is the physical layer. 13 (Refer Slide Time: 00:59) What is the physical layer consist of? It consist of wires and equipment of that nature. What is its job? Its job is to carry the bits from one end to the other. So, the concept of physical layer you have these ideas of modulation techniques and you know how to transmit the information, how to transmit the bits. So, that is what the physical layer concerns itself with. (Refer Slide Time: 01:25) On top of the physical layer is, what we call the link layer. So, what is the link layer? The link layer now has to concern itself with transmitting what are called packets or segments along the physical layer. For example, let us say you are using a wireless network, it may be that there 14 are multiple machines are accessing the same physical layer. In this case the physical medium is the air and the physical layer actually deals with how are the bits transmitted over this medium. Now when multiple people are accessing the same physical layer, there has to be some protocol by which people can decide who waits to speak when. So, this idea is called medium access control, how to control access to the medium. Secondly, there is also the idea of ensuring that when I have send data to the other end of the wire or from the laptop to the access point, I should know that my data has reached to the other end. So, there is a notion of acknowledgements. So, these ideas are what the link layer concerns itself with. (Refer Slide Time: 02:38) What comes above the link layer? It cannot be that everything is just one link. So, automatically we need the notion of being able to have multiple links which are connected with each other in order to form a network. The moment we do that we automatically come up with the idea of packets, saying that I need a packet, now this packet has to go from a source which is at one end of the network to a destination which is at another end of the network and in between there could be multiple links. There could be links which are travelled over wireless, there could be links which are travelled under the sea and there could be links which are travelled on different types of physical media. So, when a packet has to traverse across multiple links, we need the routing layer or the network layer to actually make sure that the packet goes from the source to the destination. So, we have 15 seen the network layer in at this level in various real life instances also. For example, you could think of your postal network as a network layer where you write the address of the destination and you put it into the postal system and the postal system has its own way of carrying the letter from the source to the destination. (Refer Slide Time: 04:09) So, the function of the network layer is very similar to the function of the postal system when you are sending one letter. Going ahead you might have more than one letter, you might have large packages which have to be sent, such as we saw in the analogy example. So, in this case what is required is that, you want to take a large document and you want to chunk it into multiple pieces, you want to segment it into multiple pieces and send each of them through the network layer. Why is this important? This important because if we send the entire large document as one chunk and let us see some part of it gets corrupted in some link of the transmission, then at the receivers end the entire document becomes unusable. So, the idea is to take a large document and split it into multiple segments and send each segment separately. So, each segment you can think of as one letter. So, each segment goes separately. Now, because each segment is going separately, it may happen that some segments get lost or it may happen that some segments reach before other segments. Hence we need the transport layer which actually keep track of this segments. So the job of the transport layer is to take the document or take the information which has to be sent, split it into multiple segments, keep 16 track of the segments by sequencing them, putting some sequence numbers on them and communicating with its peer at the other end in order to ensure that which sequences have been received, which segments have been missing, which need to be retransmitted. So, there is a notion of an acknowledgement. So, this broadly is the work of the transport layer. (Refer Slide Time: 06:01) On top of the transport layer is the application layer which is what we are familiar with as users. So, when you open your browser essentially what you are doing is, you are opening an application layer. When you connect to a web server and you search for information or you are downloading something, you are opening a connection between the application layer on your machine and on the server’s machine. So, what is the information that is being passed on at this layer? The information is that you are sending a request and in response to that request, you are getting back some information. So, the client makes a request and the server sends the response. So, this information is at a very abstract level, it does not worry about what is the way in which it goes or what is the link layer and so on. So, the client simply says give me this document, the server takes the document and gives it to the transport layer on its side, which then segments it, sends it through the network and it comes back to the client side. So, this is the way in which communication happens on a network. 17 Now that we have understood it conceptually let us look at how it actually happens in a real network. So, we will use a tool called wire shark, which is able to capture these packets and open them for you to see what is there inside the packets. 18  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 06 Layered Nature of a Network (Refer Slide Time: 00:31) So now, when we move from the analogy to the network, we see that there is a concept of layering. Just as in the analogy we had the manager and then we had the dispatch section and then we had the packaging section, in a network again there are multiple layers each layer has its own functions. 19 (Refer Slide Time: 00:42) So, before we get into that, let us ask the question why do we need these layers? what does it give us? So, the key point here is that it can be done without having layers also; however, in that case every application. For example: you might think of a web service application; let us say, you are using your web server to get information from a client or you might think of a file transfer application, where you are moving files across the network. So, each application would have to do the entire packaging and the entire dispatching operations by itself. (Refer Slide Time: 01:16) 20 In order to avoid that, what we do in networks is to provide this idea of layers, so that the functions can be separated. So, the application layer now only worries about talking to the corresponding application layer on the other side of the network. The transport layer worries only about ensuring that packets are sent in the correct sequence and are received in the correct sequence. The dispatch layer worries only about making sure that the packet goes along the desired route. (Refer Slide Time: 01:48) So, by separating these concerns it becomes easier to manage the applications as well as to develop applications quickly. 21 (Refer Slide Time: 02:00) Now, how do these layers work? The basic idea is that each layer provides a service to the layer above and it utilises the services of the layer below. For example, the application layer gives some data and some address to the transport layer. Now, this information is the transport layer’s responsibility to communicate from one end of the network to the other end of the network.At the other end again this information is reassembled and sent back to the application layer. So when you make a web request, what happens is your request is packaged and sent across the network at the web server side, the request is again received in the original form by the web server. So, each layer provides a service to the layer above it, through what is called the interface. The interface is nothing, but sending of the data and some control information from one layer to the other. 22 (Refer Slide Time: 03:11) Now, along with layers there is the concept of protocols. What is a protocol? Protocol is nothing, but an understanding between two entities of how they are going to communicate. So, there are various protocols. In the networking world, the protocol means that a layer on the client side is communicating with the corresponding layer on the server side. For example, when the transport layer which is responsible for creating the packets and numbering them and keeping track of them is communicating with the peer layer, the transport layer on the other side of the network, they need the notion of acknowledgements. 23 (Refer Slide Time: 03:52) So that the layer can keep track of which packets have been received on the other side, which acknowledgements have come, which packets may have been lost, which need to be retransmitted and so on. So, in order to have these understanding between the same layer across the network that idea is called a protocol. So, they have a mechanism by which certain control packets are exchanged by which they come to know, what is the current state of the data that is being transmitted across the network. (Refer Slide Time: 04:27) 24 To summarize the concept of layering involves two ideas. One is the idea of interfaces through which one layer is able to talk to the layer below it and pass on the data and control information. And, the other idea is that of the protocol, wherein one layer is able to talk its peer at the other end of the network. So, both of these have a notion of some syntax, which is how is the packet to be framed, what should be the control signals and so on. And some semantics, which is, how should these signals be interpreted. 25  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 07 Listening to the Internet chatter So now, you have seen there is a lot of communication that has over the internet, lot of applications trying to connect to their servers and the servers setting responses. So, your laptop itself has a lot of applications apart from the web browser we choose the internet. Now it would be really interesting to see if we could see this actual communication or as we say the chatter happening over the internet between all these devices. And yes, there is a software that can help us just do that, And that software is Wireshark. (Refer Slide Time: 00:31) So, basically Wireshark is a software that allows us to capture live internet traffic. And when we say live internet traffic, we can actually see the layer by layer architecture of the traffic, how each layer has its own information and that information later allows the communication to happen. Now, let us go and look at how Wireshark works. 26 (Refer Slide Time: 00:59) Now, I am going to open Wireshark. Wireshark is a free software you can download it, once you have installed it, just double click and once you double click it will look for the different type of networks that are available on your computer. For example, on this laptop we have a Wi-Fi card. So, I will select my Wi-Fi as the network interface that I want to monitor. And once I have done that, I will click on this blue colored fin to start capturing the communication. (Refer Slide Time: 01:29) So, as I click here what you see is that a number of packets are being shown, these are basically the communication that is happening from my laptop to different servers. 27  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 08 (Refer Slide Time: 00:01) We can see how does these different applications communicate like for a web page, how is a request from browser sent to a website and the website server response to the request by fragmenting the packets and then reassembling it. (Refer Slide Time: 00:19) 28 For the ease of looking at the packets or trying to look at the information that is available in the packets, what I will do here is, I will add a filter here which will allow us to only see packets of websites. (Refer Slide Time: 00:33) So now, what we see here is are different packets that are responsible for a website’s communication to happen. So, further we will be learning about protocols like HTTP and TCP and how they enable these websites to exchange information. So, while I am doing this what I will do is I will open up Google chrome. So, this is the dummy website. (Refer Slide Time: 01:03) 29 So, as I click here what we see on Wireshark is a lot of other packets are being captured here. Now, all these captures are relevant to the websites traffic that was just requested by us on the browser. So, the http is the hypertext transfer protocol which is responsible for the communication between a browser and a web server. So, let us click on this. (Refer Slide Time: 01:25) So, what you see here are different layers of this communication or this piece of communication which we call packet. So, you will come to know more about packets in the further course. Now, what we see here different layers of this communication, now each packet is actually divided into different layers. So, you have physical data link layer, then you have network layer and the transport layer. 30 (Refer Slide Time: 01:51) So, here what we see is the different information that is relevant to each layer like you have IP addresses in this layer and so, we come to know about these in detail as we go ahead with the course. So, what here I want to show is this hypertext transfer protocol request. (Refer Slide Time: 02:05) So, basically it says we had requested for this particular website. Wireshark tells you that the response of this request was seen in the frame 223. So, here are the frame numbers and let us scroll down to the frame 223. So, what we expect to see there is, there is another HTTP as we have seen earlier. 31 Now, this is the response to that request and there are lot of frames that we see in between. So, basically all these frames have been received to complete this entire communication and so, what does come in this communication? Here, let us open this packet and see what data was there. (Refer Slide Time: 02:47) So, what you see here inside is actually the entire web page. So, it is the code of the webpage, the entire code of the webpage that we had requested. So, we could look at it by right clicking on the webpage and viewing the webpage source. (Refer Slide Time: 03:05) 32 So, this is exactly what we had received from the server. So, this is how the entire web page has been sent to a laptop. (Refer Slide Time: 03:23) Now, how did this happen? Now before this entire response was received, we see there are a lot of packets that were received. So, all these were small-small packets which were sent to the laptop and here they were reassemble using their TCP information some something like a sequence number. We will learn more about it in the further course. So, this is how we can capture and look at live information on any of the laptop using Wireshark. Thank you. 33  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 09 Cisco Packet Tracer Welcome back. So, we have learnt a lot about networks still now, one of the things with learning anything technical is to be able to try out. The best part is if you are able to try it out in real setting, real life setting, but as we know networks are something very huge and the devices itself are not easy to get. So, one of the ways you can do it is by using a simulation software. This actually gives you a very good hands on experience on how networks works. So, the simulation software that we have chosen to use is called Cisco pack tracer and we will just give you a brief introduction on what actually packet tracer does. (Refer Slide Time: 00:35) What is Cisco packet tracer? Cisco packet tracer is a network simulation software that provides virtual experience of working with these things and when I say virtual experience though its virtual it is very close to what the real deal is. So, you get to see how the routers look like the kind of interfaces they have and you actually get to see the command line interface as though if you would have logged into the actual router. And, with the number of devices that are available in Cisco packet tracer you are able to create as many as networks, as huge as networks 34 as possible and configure a lot of different type of protocols, try different services and a lot of things. Cisco packet tracer is actually a playground. Every time you get to learn something new you could just go back to Cisco packet tracer and try it out. So, there are a lot of ways and lot of options that you have. Yes, it is from Cisco so, most of the devices are the Cisco routers and Cisco switches. Let us go ahead. First look at how do you actually go ahead and get Cisco packet tracers. (Refer Slide Time: 01:35) So, the one really good thing of Cisco packet tracer is it is a free software. What you have to do is, register at Cisco network academy, which is an academy by the company Cisco. In this academy you get a lot of courses on networking and new technologies that are close to networking, like internet of things and all of those. 35  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 10 Modes of Cisco Packet Tracer So, we will just open one of our previous files. (Refer Slide Time: 00:07) So, here you can see there is a set of devices that you can see here, there is a router here, there is a laptop, there is a PC, all of them are connected through multiple number of devices and each of them are connected to each other. So, this is how basically a network looks like on packet tracer. So, the other thing that we wanted to look at is this, which is connections. So, this is something called auto connect, you can use this to connect two different devices and it will automatically decide which kind of cable is the best. But for example, if you have a router which has Ethernet as well as fibre optic ports, you could choose between a fibre optic cable or a Ethernet cable to use as a communication media. 36 (Refer Slide Time: 00:55) Now, going back to the properties that Cisco packet tracer has. So, one of them is called the modes. So, under modes what we see is, there is a real time mode and a simulation mode. Now to see what it means, let us go back to packet tracer and see how they are different. So, what you see is a real time mode. In real time mode communication is happening, but if you switch to simulation mode, it opens a window here. So, in this place if you send a request from any of these laptops to say any of the devices on the network, it will actually show you hop by hop communication of all those packets. So, in the simulation mode you will be able to trace the entire communication from hop to hop, where as in real time mod,e you will be able to verify if the communication is on real time, you would not be able to see it instance by instance. This is what is a logical representation of a network. 37  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 11 Cisco Packet Tracer (Refer Slide Time: 00:01) The first step that we have to do is actually, go to Cisco network academy page, do a registration and then we will be able to get the free software. And along with the registration, you also get a free online course about packet tracer and it is a 10 hours long course, but even if you just see the first or the second models, they are more than enough for you to go through all this course. Let me take you to the net academy webpage and let us see how it looks like and how we can go ahead and get Cisco Packet Tracer. 38 (Refer Slide Time: 00:29) So yeah, once you log on to ‘netacad.com’, this is basically Cisco’s network academy page. So, you can go to courses and here you have packet tracer. (Refer Slide Time: 00:47) So, when you click on Cisco packet tracer, it tells you a lot about what packet tracer is and it actually is used for a lot of Cisco certifications in networks, like if you plan to work in network roles. So yeah, lot more details about what Cisco packet tracer is. 39 (Refer Slide Time: 00:53) So now, to download Cisco packet tracer, you could enroll and download packet tracer, just click this and it brings you to this page. (Refer Slide Time: 01:11) So, this page actually shows you if you enrol, you will get a 10 hours long self paced introductory course to packet tracer. I mean you do not have to do it you can still download the software and keep trying it out, but it is a good course to go through. And to do that, you just hover up on sign up today, click on English and you will be brought to this page. 40 (Refer Slide Time: 01:37) Here you enter your name, last name and all the relevant details and then you will be able to login to my net academy. So, what I will do is, I will login into my account and show you how we can download Cisco packet tracer. (Refer Slide Time: 01:55) So, once you have logged into your account. So, I have already enrolled in the packet tracer introductory course. 41 (Refer Slide Time: 02:03) So, its again as I said, it is not a mandatory course that you have to do, at least for this course, the current course that we are going through, we will be talking about all the things that you have to do on packet tracer will be showing you those. But if you are interested and if you want to play a lot on how networks work, configure different networks, this is a good place to go to ok. So, once you are here in the courses, you can go to student resources. And under student resources, you can download the latest version of packet tracer from here (Refer Slide Time: 02:31) 42 Yeah so, I already have packet tracer installed. So, I will not be downloading it right now. So, now, let us look at what packet tracer actually looks like. So, this is what packet tracer looks like. So, once you log in, this is the screen that you have. 43  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 12 Logical and Physical Typologies in Cisco Packet Tracer (Refer Slide Time: 00:01) Going back we also talked about the topology which is the logical topology and the physical topology. Now, let us go back to packet tracer and see what it means. So, the logical topology is kind of a map of the devices how they have been connected, but Cisco packet tracer also offers you a way to physically configure those devices. So, let us see now this is the map of our campus and what we have here is a building. 44 (Refer Slide Time: 00:21) So, if you click on this building it shows you an office layout. (Refer Slide Time: 00:29) So, what we have here, we have drawn a campus uplink which is basically your computer centre, where your internet service provider is giving you the internet connection. So, if we click on that what we see is a rack. So, this is what a network rack looks like and on those racks you see different devices and these devices have been represented here through these symbols. So, it is the same devices represented in a physical form. 45 (Refer Slide Time: 00:43) So, the Cisco packet tracer also offers you a way of being able to visualize your network as it would have been in the physical space. Now, we can go back from that wiring closet, go back to the computer center and look at the campus again. So, that is the difference between a logical and the physical topology. Logical topology basically shows you how different devices are connected, in a physical topology you also see where they have been placed. So, those are some of the features that Cisco packet tracer offers. Now, as we go ahead during the course, we will be showing you different things you could do on Cisco packet tracer and using those videos, you will be able to explore Cisco packet tracer. As per as a course is concerned you need not go to through the entire introduction to packet tracers course, we will be telling you what all commands that you need to solve the problems or the assignments using the Cisco packet tracer. And if you get interested and we would rather encourage that you go ahead and explore what you have learnt. Basically, design all the kind of networks you have in mind, think about your college network or think about any network and try to imagine how one would design such a network and that will definitely give you a very rich experience about computer networking. 46 (Refer Slide Time: 02:19) Go ahead and have a nice time. 47  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 13 Devices on Cisco Packet Tracker So we see here, there are lot of options in the Cisco Packet Tracer. So, let us look at them one by one. (Refer Slide Time: 00:07). So, basically there are lot of devices, among devices you have network devices, you have end devices, end devices are usually computers, servers that basically communicate over the network and the network devices are the ones which make communication possible. Then you have new kind of devices like IoT devices, smart sensors, smart doors, smart furnaces and all of those kind of devices. Then each device has a different kind of interfaces, like the physical interface, the user and the configuration interface which we will be looking at soon. Then there are different kind of connections. By connections, we mean wires. So, we have optical fibres, we have lan cables which are basically twisted copper wires and then on the simulation engine itself, which is our Cisco packet tracer, we have modes of simulations. Now, we will be talking more about these as we go further. So, let us go back to packet tracer and let us look at what are the different devices and how what are the different interfaces there. 48 (Refer Slide Time: 01:09). So, if you look on the bottom left corner of the screen. So, you can see network devices, all these icons here, end devices, network devices. So, when you click on network devices there are certain routers which are shown here. So, all you have to do is just pick one, drop it here and that is how you place a router. If you click on the router, you will be able to see the router here. So, let us zoom in and see how it looks like so, here is a switch, here are different ports of the router. So, this is what we call the physical appearance of a router and you could choose between different kind of interfaces you want to put on the router. For example, we have this empty slot here and we have say, we want four more Ethernet ports. So, we can just drag and drop so, you need there to switch off the router before adding any of the ports. So you switch off the router, you add these ports. They get added. So, now, you have four more ports on the router. So, this is how actually, this is original Cisco router looks like, there are number of them, different models over here. Then you have the different windows that we were talking about, so one of them is the configuration window, yeah. To do any configuration, the router must be switched on. So, as you go into the configuration window, what you see is these shrilling hashtags, so that means, the router is booting up. So, this is how a router’s command line interface actually looks like. So, if you login to a router using a console cable and use a terminal on your PC so, that is how basically how a router looks like. So, here you have all the different settings of a router, 49 the router’s name, host number, the different type of routing properties, the different network addresses, all the things you could configure inside those and then you have the command line, which actually simulates, how a router’s command line look looks like. (Refer Slide Time: 03:01). So, let us go ahead and close this and let us look at say an end device. So, we can pick a PC over here. Drag it here and when we click over it, you see the physical appearance of the PC. (Refer Slide Time: 03:11) 50 Then, if you go to the configuration here, you can change the settings of the network adaptor of the PC. (Refer Slide Time: 03:25). Then, if you go to the desktop, it gives you all the kind of softwares that are available usually on desktops. For example, you have this place where you can configure the IP address of the desktop. This is just like a standard web browser. So, you type a URL and based on the URL, it will go and contact the server in your virtual network. (Refer Slide Time: 03:39). 51 So, why not let us go and see actual network that has been configured on Cisco packet tracer. 52  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 14 Introduction to the Cisco Packet Tracer Activity for Week 1 So, let us use Cisco Packet Tracer, the network simulator which will allow us to experience how packets flow through a network. Remember, we were discussing about explaining how a webpage loads to our friends who were not used to computer networks. Do you think this Cisco packet tracer will help them to see what happens behind the screens? Yes definitely. So, let us go to Cisco packet tracer and see how the packets traverse from your system to the server. 53  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 15 Introduction to the campus network on Cisco Packet Tracer Hello and welcome back. Now, you have seen an introduction to Cisco Packet Tracer. It is a good time to actually go ahead and look at how we can request a website from an server and how does this request go through the entire network and how is the response received back from the server. (Refer Slide Time: 00:21). So, we were talking about the communication being sent in fragments as we also saw in the analogy, that the orders were being delivered in small packages and whenever there is a huge request, say a very huge file to be sent by the server back to the laptop which made the request, it is sent in small packets. So, how do these packets actually reach the destination and how the laptops actually take all these packets and finally, come up with the website that was requested? So, let us go to packet tracer and see how this happens. 54 (Refer Slide Time: 00:57). So, here we have a topology that has already been laid out, it is just a imaginary topology of say a campus network. This is the logical representation of the topology. (Refer Slide Time: 01:11). So, if you look at the physical one, here is a map. So, this is a campus map. Here what we see is the internet service provider and the internet service provider is linked to the computer centre which is again link to a hostel. We could go inside the computer centre and look at certain devices that are available there. 55 (Refer Slide Time: 01:33). And similarly if we go back to the hostel, we can see there is a network room for the hostel where certain devices are placed, there is a Wi-Fi modem here and we have a laptop, a cell phone and a desktop which is connected directly to the wall socket. So, these are certain scenarios that usually happen in a hostel. So, when we look at it logically so, these are the laptops and the cell phone and the PC that we were talking about, that this is the Wi-Fi router that we saw in the lobby of the hostel. And similarly this entire network belongs to what the hostel network is and the hostel network is then connected to the campus network and the campus network has its own devices like this server. So, what we will try to do now is, we will try to request a website which is there on the institute website server from the hostel network. So, let us open the laptop and let us go to the browser. 56 (Refer Slide Time: 02:31). So, name of the website is ‘insti.ac.in’. Now, if we hit ‘Enter’, we see this dummy website coming up, which has a link to a campus map and IP address allocation of the institute and when you click, click on campus map we see this image opening. (Refer Slide Time: 02:45). So, right now what we saw is the laptop has sent the request and the server has responded. But how do we see it step by step? So, what we can do is, go back into the simulation mode and then try. 57  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 16 Loading the page in Simulation Mode (Refer Slide Time: 00:01). So, what we will do is from the real time mode, we will switch to simulation mode. So now, what we will see is the request for their website from our laptop going to the web server. So, let us go ahead and place that request. So, I will just press ‘Go’ again and now I will click on ‘Play’. 58 (Refer Slide Time: 00:23). So, as I click on ‘Play’, you see these small packets have started flowing from one device to the other and eventually find the way to the server and the server is trying to respond to it. So, we just saw, how the packets one by one were going to the server and back. Now, since this was the very small website, so it was just loaded in one go. So, what we will try to do is now, just try to click on the campus map link which has an image, which is a heavier file than compared to the text file we just loaded and let us see how it handles that. So, we will go to the simulation mode and we will try to request the campus map. So, as we click on ‘Campus Map’ and then we hit on ‘Play’, we see certain traffic has started flowing again. It goes from the laptop through the network and reaches the institute server. The server response goes back to the router and to the laptop. Now, let us see what is there on our webpage. 59 (Refer Slide Time: 01:29). We see that the webpage has loaded, but the image is not loaded yet because the image is being communicated. Now, what you see is, there are number of packets, which are the small purple packets, which are leaving the web server and try trying to get to the laptop. So, what is happening here is, the image has been broken down into small fragments and these fragments are being sent to the laptop, one at a time. So, all these segments are of a particular length. So, until and unless we are able to create the entire segmented form of the image, this communication keeps happening. So, what we will do is, let us switch to real time and see the image load. So, we have here switch to real time and what we see is the image has loaded. 60 (Refer Slide Time: 02:21). So, now what we can do is, go back to the simulation mode and try to see what, are the different type of information that are available. 61  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 17 Inspecting the packets in Simulation Mode (Refer Slide Time: 00:01). So, now, what we can do is, go back to the simulation mode. So, will I go back. I will try to reload this web page and hit run play. (Refer Slide Time: 00:11). 62 We let a few packets go around. So, we have a packet going to the hostel router and then to the switch and then to the server. So, in the future, we will see how these packets are able to determine on which path they are suppose to go and here what we see is one of the packets reaches the phone also, but the phone rejects it because it was not requested by the phone. So, the second round of packets have started coming up and now we see a stream of packets has started going from the web server directly to the laptop. So, what we can do is, we can pause this here and let us try to look at the information that is inside these packets. So, let us see the first one from my laptop. (Refer Slide Time: 01:01). So, as you click on this, you can see this information that is available here. So, what you see here is the layered architecture of the communication and it says there are certain information like IP header source, there is some address over here and there is a destination address over here and then again there is another type of address called the MAC address over here and another is the destination MAC address over here. 63 (Refer Slide Time: 01:29) So, if you want to look at how the packets look actually, this is how they look. Now, let us close this one and we will go down and try to see what happens at the institute web server. This was the first one which said 200 content length was 2016, that is when the web page just loaded. (Refer Slide Time: 01:39) Now, let us go back and see what is the information that is available on packets which were streaming out of the issued server. So, what we see here is, there is a huge sequence number. Earlier what we saw was, the sequence number was 1 and the acknowledgement number was 1. So, now, what we see here is the sequence number is huge, that means, there are these many 64 packets which will have to be sent to send the entire image and the laptop has already received these many images. This way the laptop is able to tell the server that these are the numbers I have received and based on these, the server and the laptop keep a track of how many more are to come. So, we just saw how communication happens over the network when you request a website and the server responds back. 65  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 18 Editing the dummy website on Cisco Packet Tracer (Refer Slide Time: 00:01). So, we will just go into the server and see where the files are stored. So, if we click on ‘Services’ and ‘http’, this is where the map file is. So, if you click on ‘Edit’, you can see, you could change certain things here and that would reflect. 66 (Refer Slide Time: 00:13). For example, I can edit this line to be “this campus map”, I will click on ‘Save’, ‘Yes’. I close it, go to the laptop, open the browser again. (Refer Slide Time: 00:41). Type the URL and as I click on ‘Campus map’. 67 (Refer Slide Time: 00:49). It shows me, image of this campus map. So, this is how you can experiment in Cisco packet tracer, you can create your own websites and try to communicate from a laptop to the server. (Refer Slide Time: 01:03). So, now the other question is, how did the laptop understand that ‘insti.ec.in’ meant that particular web server? And how was it able to reach there? We will be talking about it in the future lectures, but just to show you, there is something called DNS or Domain Name Service. 68 (Refer Slide Time: 01:19). Here, this particular string or the name has been mapped to a particular IP address which belongs to this server, we can see it here, ‘10.10.10.10’. We will look at it in more details in the future lectures. (Refer Slide Time: 01:31). So, we encourage you, that you download this packet tracer file, try to make some changes and see how communication happens. That is it for now. Thank you. 69  Demystifying Networking Prof. Sridhar Iyer Indian Institute of Technology, Bombay Department of Computer Science and Engineering Lecture – 19 Summary of the Cisco Packet Tracer Activity We just saw, how information is captured and fragmented and sent in packets in Cisco packet tracer. And we also saw, how packets were formed in the network and how the packets traversed hop by hop to reach to the server and back. 70  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 20 Introduction to Anupam’s Adventure So, this was a nice way of knowing that the network works in a layered structure and each layer takes care of its own function and also provides the functionality to the next layer. Yeah and broadly we have seen how a packet travels through all these layers to reach the destination, doing hop by hop communication. Now the logical question would be how do these packets nowhere to go? This brings us to the question of concept of IP addressing. So, the concept of IP addressing, actually is a fairly detailed concept and since in this course we have dealing with it at a abstract level, let us start with looking at this story called Anupam’s adventure. So, you can go and now read the story which gives you the idea of how IP addressing works and then come back. 71  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 21 Anupam’s adventure brings us to IP Addressing We saw that Anupam had some wonderful adventure, especially when he had to connect that the numbers given in his chit was actually IP address and then follow step by step instructions to reach his destination. The other interesting thing that the story explained was, four parts of an IP address and each part takes a number from 0 to 255 and creates a unique identifiable address. At this point, we have a reflection question for you. You have to stop, read the question, note down your answer and then continue with the learning dialogues. (Refer Slide Time: 00:44) The question is which among the following is a valid IP address? Option a: 192.168.298.2; Option b: 10, 23,46,01; Option c: 35-45-66-00; Option d: 127.0.0.3 So, those of you who had chosen option d, yes, it is the correct answer. 72 (Refer Slide Time: 01:23) So, when we look at option a; option a has a number which is greater than 255. So, it cannot be a valid IP address. For option b and c do not use the dotted decimal notation, they have commas or hyphens. But there is more about IP address in than just the dotted decimal format. Let us say what sir has to say about IP addressing. 73  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture - 22 Addressing at various layers Now, when it comes to addressing, what we find are that there are primarily three categories or three levels of addresses which are important. (Refer Slide Time: 00:11) One is called the physical address or the MAC address, which is essentially the address given to the card which is communicating with the link layer. So, in your computer if you have an Ethernet card, there will be a MAC address; if you have a Wi-Fi, there will be some other MAC address. So, that address is assigned by the manufacturer to identify that card. Is it enough to have only that address? If we have only that address, what might happen is that in order to identify where is a machine, it can get very complicated. Right? I mean different machines will go to different countries and they will be in different locations and in order to send a packet from a client, which has such a MAC address to a server, which is also having only such a MAC address, is going to become very complex in order to make sure to get the packet from the source to the destination. 74 (Refer Slide Time: 01:13) So, the MAC address is useful at the link layer where multiple machines are using the same link to communicate or when machines are on a LAN or a local area network. What happens when we go beyond the local area, when we go beyond a LAN? Some mechanism is needed to categorize these addresses, so that packets can be routed. It is very similar to the postal system, where we have the notion of cities and streets and buildings and flats. Similarly, we need a notion, a hierarchical notion of addressing. So, that packets can go from a source to a destination. These addresses are called network addresses or IP addresses. IP stands for Internet Protocol. So, these addresses are used to take a packet from the source to the destination. 75 (Refer Slide Time: 02:13) On top of it, is it possible that we will always be able to remember the IP addresses of different machines? For example, what is the IP address of your favorite search engine, you do not remember these addresses and since the machines may change IP addresses may change, it is required that for humans to be able to remember what is easy, are the logical addresses. So, that is the third level of addressing which is what we use as ‘www.et.iitb.ac.in’ or any other address that you are familiar with. 76  Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 23 IP Addresses (Refer Slide Time: 00:02) Now, let us look at this figure, in the centre you have this box. This box actually represents a router. What does a router do? Router is an end point of a network or in other words, it connects two different networks to each other. Router may have multiple interfaces, but on each interface it is a separate network. So, you can think of a router as a post office where the packets from one area come and then they are looked up and sent to other areas along different routes. So, let us look at one side of this network. So, on one side you see addresses of the form 10.1.1.2 and so on. At the bottom we see that an IP address is nothing, but a 32 bit number. Ok. So, it is a 32 bit binary number, which for ease of human comprehension, is split into 4 octets. So, and each octet is now read in the corresponding decimal form. So, that is how one side of the network becomes what is called the 10.1.1 network. So, the network is often referred to using the name that is given to that router’s interface. So, 10.1.1.1 is the router which is catering to the entire network of the other IP addresses that we see. So, each of these can be machines or they can even be local networks inside that. We will see how that happens, a bit later. 77 So, what this router is doing is, its connecting the 10. network to the 223. network. So, what does a router have? Suppose you look at the machine which says 10.1.1.2 wants to send a packet to the server which is at 223.1.5.2. So, essentially what will happen is, that the machine which is there at one end of the network, it knows where to send the packet in order to reach the router. In the router there will be a table which will say that if a packet comes on this line, with this IP address and wants to go out to that IP address, then it has to be sent out on that line ok. So, this is a very conceptual level of explanation. Essentially, the routing table is simply mapping the input link, the input IP address to the corresponding output IP address and the output link. So, the router knows which is the link on which to send the packet or forward the packet, so that it goes towards the 223.1.5.2 machine. While this is a very simplistic example we have only considered 1 router and 2 interfaces. The same idea keeps on extending in order for us to reach from anywhere in the internet to anywhere else in the internet. 78 ‘ Demystifying Networking Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 24 Address Translation Now, while we have seen this idea of IP addresses at a conceptual level. There are lot of intricacies which are important in order for the internet to function. At one level, there is the idea of being able to translate one address into another. For example, we saw that there were three levels of addresses, we saw that there were MAC addresses and then there were IP addresses and then there were logical addresses. (Refer Slide Time: 00:28) So, who translates a logical address into an IP address? That is called the domain name service. So, there is a server whose job is simply to, given a domain name which could be ‘www.iitbombay.ac.in’, to come back with the IP address for that domain. Now the IP address itself, we can see that, there could be multiple IP addresses for a given domain and so on. So, there will be one machine or a group of machines to which the domain name is mapped to. So, this domain name server itself is a distributed entity, because we cannot have a single domain name server for the entire world. So, just as the IP addresses are organized hierarchically, the domain name service is also organized hierarchically. So, there will be a domain name server for an organization and then 79 ‘ there might be one for the country and there might be a network of these domain name servers across the internet. Anyway the basic idea is that the DNS or Domain Name Service is used to translate from the logical address to the IP address. (Refer Slide Time: 01:49) Now, once you come to the network, there is the notion of translating the IP address to the MAC address. Because remember it is the MAC address which is seen on the link. So, who does that translation? Once again here there is a different protocol that is used, that is called the address resolution protocol. This is a very simple protocol, and this essentially involves saying that who has this IP address. So, essentially what the router does or whichever is the sending entity, it transmits into the entire local area network as to who is this IP address and the machine which has that IP address, responds saying tha,t that IP address is mine and here is my Ethernet address or my WiFi address or whatever, MAC address. So, in that way, the machine to which the packet has to be sent is identified. 80 ‘ (Refer Slide Time: 02:51) Going further, we see that there are some addresses which have to be kept within an organization, right? For example, IIT Bombay may have 1000s of computers. We do not want each of these computers to be visible on the internet. We do not want each of them to be listed in some DNS server. So, IP addresses in that sense are classified into what are called public IP addresses and private IP addresses. Private IP addresses are what an organization uses within itself for communication within machines inside the organization and then the public IP addresses are the ones which the organization uses to communicate with the rest of the world. So, now, imagine that you are sitting on your computer inside the organization and you are accessing a web service. How does it know that which machine has originated the request and where to send back the reply to? So, as you can imagine there would be a router like entity, in this case, this is called the Network Address Translator or NAT which basically has the job of keeping the mapping of which IP address, internal IP address, has been sent out on which link as the public IP address. Well all these may sound a little complex in the abstract, it is actually fairly straight forward. So, it is a very hierarchical mechanism and most text books have lucid explanations of these topics. You can refer to some of these textbooks, there are lot of videos also available which you can look at, to get a more in depth understanding of each of these concepts. 81 ‘ (Refer Slide Time: 04:50) So to summarize, addresses are at three levels. One is the MAC address, which is assigned by the manufacturer, then is the IP address. The IP address is, two entities are responsible for the IP address, one is the Sysad of your organization, the system administrator of your organization, which might be your IT team which assigns the IP address within the organization. That is the private IP addresses and then there is the ISP, who assigns the public IP address. So, there is an authority which decides which IP address space can be given to which organizations, which are the ranges of IP addresses that can be given to a particular organization and so on. So, often these authorities work with the ISP to assign the public IP address for an organization and then on top of it is, the logical address which is your domain name and that is provided by the domain name registrar, which again through a network or a series of domain name registries, allows you to purchase or to procure different domain names of your choice. So, in summary, the domain name is then associated with the IP address which is then associated with your MAC address and that is when your machine becomes visible on the network. In order to see how this happens in an actual network, we will now see a simulation using packet tracer. 82  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 25 Introduction to IP Addressing Hello and welcome back to the new video on Network Addressing. So, in this video we will be talking about IP addresses mainly. (Refer Slide Time: 00:13) So, what are IP addresses? IP addresses are basically certain addresses which are used to communicate between two devices. Now these addresses define which device you are communicating from and which device you want to communicate it to. Like normal addresses, they help the packets or the communication determine where it is supposed to go and where it is supposed to place the request and get the response from. So, when you look at IP addresses there are typically three things we talk about, they are IP addresses themselves, something called the subnet mask and then a gateway. So, if you have ever try to configure or enter an IP address into any of your devices, say your laptops, you would have seen these three things. So, what do they mean? So, IP addresses as we know this is a unique address that your system has over the network. Now, when we come to subnet mask, this basic address or these digits define, whether this particular IP address and the others belong to a single network. 83 So, using this a computer can calculate, whether 192.168.0.2 or any other IP address with this subnet mask belong to the same network or not. How? We will look at it in sometime. And gateway is the IP address of your router. So, what is a router? Router is a device that actually divides networks or marks the end of a network. So, the function of a router is to connect two networks. So, so the gateway is the router of your network. For example, at your home if you have a WiFi router, that router connects you to the internet service provider like different company which give your internet. (Refer Slide Time: 01:53) So, what IP addresses are? They are set of 4 digits as you saw or we call them 4 octets. Why octets? So, as we know computers communicate using the 0s and 1s language. So, each number is represented by 8 digits, which can be 0 or 1. So, here what we say is 192s representation in the octet form which is basically binary is 1 1 and all the 0s. 84 (Refer Slide Time: 02:26) So, how do you get that? So, if you look at this. So, you take 2 to the power 0 which is 1, 2 to the power 1 which is 2 and similarly if you calculate all of these and then multiply whatever value here with the value here. So, this becomes 128 and 64 and then you add all these that should become 192. So, this is one, I would say a crude way of calculating what the respective binary values are or how you can get a value from the binary value. Now, let us look at the other a number which were there in the address, which were 192, 168, 0, 1 and 255. So, these are the representations of the other numbers. So, an octet can go from 0 which is the minimum number it can represent to 255. 85 (Refer Slide Time: 03:16) Now, what does this mean? This means that an IP address in each octet can take a number between 0 to 255, which is 256 numbers. Similarly for the other octet as well. We can have at the max of 255 to the power 4 of number of unique addresses using this addressing scheme. But do not you think these are these addresses are bit less in number if we consider the entire internet? Yes, definitely. So, we have something called IP v 6 that has come in and that is a totally different topic that to be discussed and there is another concept of something called public and private addresses that we will be looking into soon. (Refer Slide Time: 04:03) 86 Now, let us get to the next part of IP addresses which was the subnet mask. 87  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 26 Creating a network with Sub-net mask (Refer Slide Time: 00:00) So basically, the subnet mask determines, I mean, which IPs belong to one network. For example, we saw 255 255 255 0 as the subnet mask for the IP address 192 168 0 1. So, how do you get to know which are the other IP addresses in the same network? As we know that, 256 are the total numbers that can be represented in this one or in this one octet, as we call it, so, if your network mask is 255, so the number of representations you can use for that particular network is 256 minus 255 that is 1. So, you could use only 1 representation, which means 192. Similarly, if we have 255 in three places, that means, all the IP addresses will begin with 192 168 and 0 belong to the same network. But here what we have, we have a 0. 0 means we can have 256 representations that would be 0 to 255 in the same network. 88 (Refer Slide Time: 01:01) So, what does that mean? That means, any address with 192 168 0 which were the single numbers allowed and the last octet from 0 to 255 belong to the same network. So, whenever a network is created, by default 2 IP addresses are reserved, which is the first and the last IP address. So, here the first IP address is ‘192.168.0.0’ and the last one ‘.255’. What we see here, is the first one which is called the network address, is used to identify the network and the last is called the broadcast address. The total number of IP addresses that you could use are all the IP addresses between 0 to 255. With the net mask of 255 255 255 0 what you can say is, all the IP addresses between 192.168.0.0 and 192.168.0.255 belong to the same network. 89 (Refer Slide Time: 01:57) So, what are the numbers that the subnet mask can take? As we saw, in case of IP addresses this could take anything between 0 to 255, but in case of subnet mask, it is different. So, a subnet mask and take any of the values between 0, 128, 192 so on to 252, 254 and 255. 90  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 27 Nomenclature of a sub-net mask (Refer Slide Time: 00:00) What are these special values? Let us look at them. (Refer Slide Time: 00:02) 91 So, how subnet mask work is, they use the binary form of a subnet mask, do a logical operation with the IP address and then try to determine the network address. But to do that, what is very essential for a subnet mask is, to have the trailing set of 1s. So, a subnet mask can only take values which are trailing 1s and in continuation. (Refer Slide Time: 00:29) Let us look at some of the other type of subnet masks. So, another subnet mask here is 255 255 255 0 which we just saw. So, it has a 8+8+8 that is 24 trailing 1s. Similarly, a subnet mask for 255.0.0.0 has a 8 trailing 1s. Similarly, a subnet mask for 255 255 255 252 would a have a total of 30 trailing 1s. (Refer Slide Time: 00:57) 92 So, using these different type of subnet mask, how can you calculate the number of IP addresses that are available? 93  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 28 Network addresses and Private networks (Refer Slide Time: 00:00) So again, as we saw here, if it is 255 here 255 here 0 and 0 here, that means, the IP addresses which could take values from 0 to 255 in this octet as well as 0 to 255 in this octet would fall under the same network. Like, 192.168.0.0 with the network address and here the broadcast address becomes 168.255.255. So, any IP address between these two IP addresses would fall in the same network, if they are carrying the subnet mask. 94 (Refer Slide Time: 00:33) Now similarly, let us look at a different kind type of a subnet mask. Here if we have 252, so, here we can only have four options 0 to 4 where, 0 and 4 will be taken up as network and broadcast addresses. So basically, you have only two usable IP addresses. So, could you guess, which would be a ideal situation to use such kind of a network? (Refer Slide Time: 00:55) Now, this is a nice time to look at a question. So, the question here is, if the network mask was 248 how many addresses would we have here and what would be the network address and what would be the broadcast address? So, pause a video for a while, think about it and once you have 95 your answer, take a note of it and then proceed. So what we saw here is, net mask of 248 would allow us 8 numbers that could be used. Now among that 0 and 7 would become the network and broadcast addresses, so, you would have a total of 8, sorry, 6 usable IP addresses. (Refer Slide Time: 01:34) Now, this is another form of representing a network mask. For example, for a network which starts from 10.0.0.0 to 10.255.255.255, you could represent it as 10.10.10.10/8 where, 8 represents the total number of trailing 0s. So, here a subnet mask would be 255.0.0.0. So, that has been represented by ‘/8’. Similarly we have ‘/12’ and ‘/16’. Now what is so special about this IP addresses? So, as I told you there are two type of IP addresses, the first one is a private IP address, the other a public IP address. A private IP address is an address which can be allocated by anyone. Anyone as in, so, if you have your home network you could use any of these IP addresses to create your home network. But the catch here is, these IP addresses are not routable on the internet. So, any IP addresses apart from these, are routable in the on the internet and those have to be taken from authorities which issue unique IP addresses. So here, we use private IP addresses, in say institutes, some organizations, even your router normally uses 192.168.0.0 as the set of IP addresses at home. So, how does it communicate over the internet? 96 (Refer Slide Time: 03:02) This brings us, two concepts that will be discussing over packet tracer. How do we actually assign IP addresses to different systems, manually or automatically? and something called ‘network address translation’. 97  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 29 Introduction to the Addressing Topology So, now you came to know about IP addresses. Now let us look at Cisco packet tracer and try to see how we can assign these IP addresses and how the different IP addresses play a different role for communication to happen over the network. (Refer Slide Time: 00:15) So, here what we have is a file called ‘Addressing’. 98 (Refer Slide Time: 00:19) If you open this, you will find your packet tracer topology ‘IPAddressing.pkt’. (Refer Slide Time: 00:30) Let us open this and see what do we have here. So, what you can see here, it opens at this network, but if you zoom out, you will see a huge network that has been displayed in this particular file. 99 (Refer Slide Time: 00:47) Now, let us look at the network very closely. So, what you see here is a set of 4 routers here. (Refer Slide Time: 00:52) And if you scroll towards your left, you see the old topology that we were using as a campus network, but instead of the cloud here, the internet which we called it, we have a set of entire network. So, what is internet? Internet is a basically, a connection of lot of networks. Now, just to give an idea what this entire topology is. 100 (Refer Slide Time: 01:14) (Refer Slide Time: 01:15) So it is a hypothetical case, where we have the campus in Mumbai, we have another company in Bangalore, one in Hyderabad and a hotel and a company in Manali and they all are connected via a internet service provider whose backbone runs at Delhi. Now, let us look at the logical view of the topology. Now, this is what is the network of the campus. 101 (Refer Slide Time: 01:40) Now, as we have seen earlier, on campus we have the computer centre and the hostel. (Refer Slide Time: 01:45) 102 (Refer Slide Time: 01:46) (Refer Slide Time: 01:49) 103 (Refer Slide Time: 01:52) So, if you look inside the hostel, you have this floor area where, you have a server room which has your networking devices and all of them are connected to a Wi-Fi router. So, this is a Wi- Fi router which is there in the gallery to which a laptop or a phone could connect. Similarly, there are wall internet ports to which PCs can connect using a wire. So now, let us look at how can IP addresses be assigned here. 104  Demystifying Networking Prof. Sridhar Iyer Department of Computer Science and Engineering Indian Institute of Technology, Bombay Lecture – 30 Addressing a local network and DHCP (Refer Slide Time: 00:00) So, if you ever have looked at a configuration interface of a home Wi-Fi router, it looks something like this. (Refer Slide Time: 00:04) 105 So, you have something called the internet setup and the network setup. So, internet is basically the outward side of your router which is in the case of home router, your internet service provider and the inward side which is where your Wi-Fi device is connect, is your local area network. (Refer Slide Time: 00:26) So, your network setup basically gives IP addresses to the devices connected to the router in the Local Area Network or LAN. And the internet setup basically gives the IP address from the service provider. Now, here what we see is, this particular router is connected to the hostel network. And, it has been given an address of 172.16.15.4. So, when we see the subnet mask here, we see that any address which has 172 16 0 to 255 here or 0 to 255 here,belongs to the same network. So, you have a lot of IP addresses available here, which is good enough for, say a network for a hostel. Now, this IP address is the outward IP address of a router. Now when we look at the network setup, so here what we have is, all the devices that connect to the wireless router and which in turn, connects them to the internet. So, all these network devices have been given a separate set of IP addresses. So, what we see here is, and a point also to notice that, no two interfaces of a router can fall in the same network. So, what the router does? It connects two different networks. So, two interfaces of a router cannot fall in the same network. Now here we see is, in the local area 106 network we have the network, 192.138.0.1 with this subnet mask, that means, we can have 255 addresses in this address, in this network address. Something interesting that we see here is, something called DHCP, a Dynamic Host Configuration Protocol. Now, this is what enables us to dynamically assign IP addresses to devices as they connect. So, here the setting says that, the starting IP addresses for DHCP would be 192.168.0.100 and maximum number of users that it could assign IP addresses dynamically, would be 50. Now, typical home Wi-Fi router can sustain up to somewhere around 15 to 20 devices. So, 50 would be a good enough number for a typical Wi-Fi router. (Refer Slide Time: 02:41) So, as soon as the devices connect to this Wi-Fi router, they are automatically assigned a IP address by the router. 107 (Refer Slide Time: 02:44) (Refer Slide Time: 02:46) Now, let us look at this device and the IP address which has been assigned. So we see here, DHCP has been activated. So, what we see here is that DHCP has assigned the IP address 192.168.0.100 with the same subnet mask and default gateway is the IP address of the router itself, which we saw when we were looking at the routers configuration page. Now, that we have seen DHCP is able to assign addresses dynamically. Now, let us see what happens when we have to assign IP addresses manually.