Computer Networks PDF

2/38 In this course, we have covered a single computer’s components as a whole. The topics that we have explored have looked at the underlying hardware architecture of a computer, the binary representation of its data and programs as well as the software structure that equips it with the ability to...

2/38 In this course, we have covered a single computer’s components as a whole. The topics that we have explored have looked at the underlying hardware architecture of a computer, the binary representation of its data and programs as well as the software structure that equips it with the ability to handle multiple tasks. In this lecture, we will look at computers that are connected to one another. These are commonly referred to as networks.  Networking In which we learn about how computers connect to one another.  4/38 Networks Computers are powerful standalone machines. But by connecting to other computers, they become even more powerful. A series of connected computers can transfer data to one another and thus are able to exchange information and share resources such as streaming media content (Netflix), software (TikTok) and data storage facilities (iCloud or Google Drive). Computers that are linked together to share information and resources form what is referred to as a network.  5/38 Classifying Networks Networks of computers can be classified in three ways; By the physical By the how By the patterns of geographical accessible the connection of the distance the network is to users. computers. network covers. Therefore, each network can be classified under all of the above depending on its reach, accessibility, and pattern of connection.  6/38 Classifying by Geography A network of computers could cover short or wide geographical distances based on its capacity and its purpose. There are four ways of classifying computers based on the area they span; 1. Personal Area Networks (PAN): These networks cover a short distance and are used for short-range communications, typically within a few metres. A good example is the connection between your earbuds and your phone. 2. Local Area Networks (LAN): These networks are usually made up of connected computers within a single building such as a university. 3. Metropolitan Area Networks (MAN): These are networks of intermediate size that could cover an entire community such as a town. 4. Wide Area Networks (WAN): These are networks that link computers that are thousands of kilometres apart, including computers that are in different parts of the world.  7/38 Classifying by Access A computer network can also be designed in a way that any computer can join it or it could be restricted to only trusted parties. There are two ways that networks can be classified based on their availability; 1. Open Networks: These are networks that have the design of their internal operations available to the public. Design here means that the standards that guide how communications occur on the network are available to the public. A good example is the Internet. 2. Closed or Proprietary Networks: These networks are owned by an individual or corporation and thus their designs are usually hidden to maintain ownership rights. For such networks, licensing fees have to be paid and contract agreements must be signed before use.  8/38 Classifying by Patterns Computers in a network connected to one another based on different patterns of arrangement. A pattern of connection is referred to as a topology. We will cover the two most popular topologies in this class. Bus Topology: The computers are Star Topology: One computer serves as connected to a common communication the central point and all other computers line called a bus. connect to it. bus These arrangements are not necessarily physical but are rather dependent on how the computers are connected (either indirectly via a communication line or directly to one another).  9/38 Protocols In order for networks to function properly and reliably, there is a need to set up rules for how activities will be carried out. These rules are called protocols. One of the key activities that require protocols in a network is the transmission of messages (sending and receiving information from other messages). If transmission is not properly coordinated, then there is the possibility of all computers trying to send messages at the same time and as result, there could be collision in execution. Two of the protocols that have been designed to handle message collisions in the network are; 1. Carrier Sense, Multiple Access with Collision Detection (CSMA/CD) 2. Carrier Sense, Multiple Access with Collision Avoidance (CSMA/CA)  10/38 Combining Networks Networks can be combined to create an extended communication system. When thinking about combinations of networks, it is important to note the topology of the networks. Networks that have the same bus topology can be combined using devices such as repeaters, bridges, and switches. Repeater: This device passes signals back and forth between two repeater buses.  11/38 Combining Networks II Bridge: This device also passes signals back and forth between two buses but it bridge perform an additional task. It checks the destination of the signal. Switch: switch This is a bridge with multiple connections that connects multiple buses.  12/38 Combining Networks III Often, there is a need to connect networks that have different topologies and standards. In this case, the networks can be combined in a way that each network can retain its individuality and design. This network of networks is called an internet. !!! An internet is different from the Internet (note the capital case). The Internet is actually a particular type of worldwide internet (note the lower case). The connection between networks to form an internet is handled by devices called routers. router router router  13/38 Process Communication Multiple computers on a network can be used to perform a single task. In this case, it is often important that these computers communicate with each other to perform their designated tasks (processes) in a coordinated manner. Such communication between tasks in different computers is called interprocess communication. There are two models of interprocess communication; Client/Server Model: Peer-to-Peer Model: One computer (client) makes a request Computers provide services to one and another computer satisfies the another as equals (that is peers). A popular request (server). An example of client/ example of the peer-to-peer model is server model is connecting a printer to a instant messaging via chat apps and network so that all machines in the competitive gaming platforms. network can have access to it.  The Internet In which we learn about how the Internet (note the capital case) is designed.  15/38 The Internet The Internet is the most popular example of an internet (i.e. a network of networks). It originated from a series of research projects in the 1960s sponsored by United States through the Defense Advanced Research Projects Agency (DARPA). The goal of these projects was to link computer networks in a way that a problem in one of the networks will not cause a disruption to all other networks. Recall, an internet is connected in a way that each network retains its individual design. The Internet was thus initially a government-sponsored research project but it has now evolved into a commercial undertaking.  16/38 The Internet Architecture The networks on the Internet are organised in a hierarchical structure based on the roles they play. These networks are constructed and maintained by organisations called Internet Service Providers (ISPs). The four tiers of networks on the Internet are; Tier-1 ISPs Tier-2 ISPs Tier-3 or Access ISPs End Systems  17/38 Tier-1 ISPs The networks at the top of the hierarchy are referred to as Tier-1 ISPs. Tier-2 ISPs These networks are few in number, Below the Tier-1 ISPs are the Tier-2 have high-speed capacity and are ISPs. international WANs (Wide Area Networks). These networks are regional in scope and much less capable than the These networks are operated by Tier-1 ISPs. companies in the communications business. These networks are also operated by companies in the communications business.  18/38 Access ISPs An Access ISPs is a network that is usually operated by a single authority. End Systems It primarily supplies access to the The devices that connect to Access Internet to individuals and ISPs are referred to as end systems. businesses. These end systems include laptops, Cable networks such as MTN and personal computers, printers, universities that provide Internet cameras, cars, home appliances. services are examples of Access ISPs. End systems are also referred to as hosts.  19/38 The Internet Software To facilitate the transfer of messages on the Internet from one machine to another, the Internet is divided into layers of software units that perform specific tasks. These layers are; Application Layer Transport Layer Network Layer Link Layer This layer consists of This layer receives the This layer determines This layer transfers the clients and servers that message from the which direction to send message from one perform their specific application layer and the message over the layer to another. tasks. This layer makes sure it is in the Internet - basically it provides the address of proper format for determines the stops the destination of transmission on the the message will make message. Internet. before reaching its destination.  20/38 The Internet Applications Due to its connectivity and versatility, the Internet has made possible a wide variety of applications. Some of the popular ones include; Sending and Making calls via the Streaming Sharing information receiving electronic Internet (e.g. multimedia content about companies mails (e-mail) WhatsApp call) (Netflix) and individuals using websites.  Cyber Attacks In which we learn about how computers and networks can be compromised.  22/38 So far, we have covered the idea of a network, that is computers that are connected to one another to share resources. Computer networks could cover a small location or the entire global, be openly accessible or restricted, and have either a star or bus topology. However, when a computer is connected to a network, it also becomes vulnerable to unauthorised access. Unauthorised access and vandalism on a computer network are forms of cyber attack. Cybersecurity is the study of protecting computers, networks and data from such attacks.  23/38 Cyber Attack Cyber attacks can affect software, hardware and human components of a computer system. Software can be corrupted or accessed by malicious third parties. Hardware designs can be exploited or the computer itself can be destroyed. Humans can be deceived or deceiving leading to system compromise. In this lecture, we will look at four popular cyber attacks; Malware Phishing DoS Spam  24/38 Malware Malicious software are software designed to achieve a nefarious or criminal purpose. Malicious software are popular referred to as malware. Malware can be used to attack a computer or network by; Directly transferring and executing the software on the target computer. Indirectly attacking the target computer by using another computer from a distance. Examples of malware include viruses, worms, trojan horses, and spyware.  25/38 Types of Malware Viruses How they work. Harmful Consequences of Viruses A computer virus is a software that Computer viruses can also replicate (duplicate) inserts itself into a program/app that themselves by transferring into multiple programs. is already installed in the computer. A lot of viruses do little more than just replicating Once the infected program is themselves. opened (i.e. executed), the virus is also executed. Some however, have devastating consequences such as corrupting data, deleting large blocks of computer storage or degrading the operating system (OS) of the machine.  26/38 Types of Malware Worms How they work. Harmful Consequences of Worms A worm is an autonomous software While viruses replicate by transferring from one that transfers itself along a network. program to another, worms replicate by transferring It starts from one computer and from one machine to another. forwards copies of itself to other computers. Similar to viruses, a worm can be designed just to replicate itself. However, a worm could have so many replications that it degrades the performance of apps on machines or even overload a network.  27/38 Types of Malware Trojan Horse How they work. Harmful Consequences of Trojan Horses A trojan is a software that is Trojan horses are designed to perform additional disguised as a game or application activities in the background. Sometimes these that is desirable to the owner of the activities start once the trojan is in the system or they computer system. could be dormant until some condition(s) is satisfied. A trojan software is willingly imported by the victim of the attack. Trojan horses could be in the form of email attachments with attractive messages. For example, emails that promise monetary gains for installing an app. Beware of emails from unknown sources.  28/38 Types of Malware Spyware How they work. Harmful Consequences of Spyware A spyware is a software that collects In most cases, spyware is used to maliciously record information about the activities on activities such as keystrokes on a keyboard in order the computer it resides and reports to extract information. that information back to the attacker. Such information could be passwords to sensitive It is sometimes referred to as sniffing accounts and credit card details. software. *However, some companies claim that they use spyware to gather user data in order to better understand their customers. The use of spyware for such purposes is an ethical debate.  29/38 Phishing While spyware is covert (secret) way of obtaining information, phishing refers to the methodology of getting information by simply asking for it. Phishing is a play on the word fishing, because it requires casting numerous lines in the hopes that someone will “take the bait”. Some fraudster could send multiple fraudulent emails to different people with the hope that one person will respond and he will make some money from them. Phishing is usually carried out via email, where the perpetrator (criminal) will pose as a government agency or financial institution. Then he or she will proceed to ask the victim for sensitive information for supposedly legitimate reasons.  30/38 Spam Spam are unwanted emails sent to an individual’s mailbox. While other forms of cyber attack such as viruses and spyware are designed to exploit the computer itself, spam is sent to overwhelm the human user receiving the message. Spam are also used a medium for phishing and sending trojan horses. A phishing spam email.  31/38 Denial of Service (DoS) Unlike viruses and spyware that are internal to a single computer, a computer can be attacked by software executed on other computers in the same network. An example of such an attack is the Denial of Service (DoS) attack. A DoS attack is executed by generating a large number of messages using unsuspecting computers on a network. This leads to overloading the destination computer with messages. DoS attacks usually target commercial companies and leads to a disruption of their activities. DoS attacks use unsuspecting computers to launch the attack and therefore, it is necessary for each individual to take adequate precautions to protect their system. Network Security In which we learn about how computers can be protected from cyber attacks.  33/38 Securing Computers & Networks The popular adage “Prevention is better than cure” is particularly applicable when dealing with cyber attacks. With attackers have become quite inventive over the past couple of years, various tools and methodologies have been designed to protect computer and internet users from malicious attackers. In this lecture, we will look at five tools and methods Firewalls Antivirus Proxy Server Cryptography Legal  34/38 Securing Computers & Networks Securing Computers & Networks Firewalls Antivirus How they work. How they work. A firewall is a program that filters Antivirus software is a software used messages passing through a network. to detect viruses and other infections. Firewalls are designed to block Antivirus software usually designed to messages coming from known remove a specific type of infection (i.e. sources of trouble or block messages a virus, spyware, trojan horse or going to certain destinations. worm). A spam filter is an example of a Example of Antivirus software include firewall. Spam filters block unwanted Windows Defender, 360 Security, emails. McAfee and Kaspersky.  35/38 Securing Computers & Networks Securing Computers & Networks Proxy Server Cryptography How they work. How they work. A proxy server is a software that acts Networks are usually attacked to gain as an intermediary between the client access to information. and a server. Cryptography is the study of sending The proxy server shields the client and receiving messages in the from malicious actions by the server. presence of adversaries. The proxy server also acts as a filter Encryption is the cryptographic for all the messages sent from the technique used to transform server to the client. messages into a coded version to prevent access by unwanted individuals/parties.  36/38 Legal Approach to Security Legal approaches to ensuring network security have been implemented to address cybersecurity concerns. These approaches have been less than perfect and are primarily affected by two key problems; Making an action illegal doesn’t prevent it. It just means that there is a legal recourse (punishment) for doing it. Different countries have standards for what is legal or illegal. This means a legal approach to cybersecurity requires collaboration at an international scale. Countries have each taken measures to combat cyber attacks. Nigeria for example established the Nigeria Data Protection Act in 2023 to regulate the use of personal data by companies.  37/38 Individual User Responsibility As a regular user, the most important prevention technique for you is to have a basic awareness of these attacks. Below are some simple don’ts that a wise computer user should be aware of; Do not open email attachments from unknown sources. Do not download software without confirming its reliability. Do not click pop-up ads on websites. Do not leave a computer connected to the Internet when such a connection is unnecessary. It has been estimated that once a computer is connected to the Internet, at least one intruder will exploit its existence within the first twenty minutes.  38/38 Recap Computers can be connected together to form networks for sharing information and resources. Networks can be combined with other networks to form more complex networks. The Internet is an example of a network of networks. When connected to a network, a computer can be attacked in multiple ways. This could be by malware, phishing, spam or denial of service attacks. Certain preventive measures can be undertaken to curb malicious access to computers and networks. These include installing firewalls, proxy servers, antivirus software, using cryptography and designing legal policies. Computer users should also protect themselves through cautious usage.