Cisco CCST Networking 100-150 Official Cert Guide PDF

Part I Foundations Computer networks are almost magical: you send an image from this device, and it appears on some other device almost instantly, even when there is no apparent physical connection between the two devices. The reality...

Part I Foundations Computer networks are almost magical: you send an image from this device, and it appears on some other device almost instantly, even when there is no apparent physical connection between the two devices. The reality is far more commonplace that it appears from the outside. Thinking About Moving Packets There is, in fact, a direct physical connection between all the devices on planet Earth (and network-connected devices in space!) over which signals are carried between devices, no matter how remote. The magic has, as it turns out, a physical connectivity infrastructure just as real as roads. This book begins the process of untangling the magic behind computer networking, from the physical through the logical. Because shipping packages is similar to shipping packets—an idea suggested by Rick Graziani—the first chapter begins by describing transporting packets as if they were physical packages. Just like a shipping network, however, a computer network does not end with physical connections, whether optical, copper, or radio transmissions through the air. It takes a lot of information to transfer stuff from one place to another. For the shipping network, the infor- mation required to get a package from one place to another is cleanly separated from the package itself. Before we can dive into computer networking proper, however, there are a few topics worth mentioning—words, thinking, and change. Thinking About Words Computer networks are messier than the physical supply chain. In computer networks, the packages being shipped are, themselves, made of information. This makes the words we use to talk about computer networks difficult; marketing often takes over where technology ends, leaving us with multiple, overlapping meanings attached to a single word. These overlapping meanings often makes it hard to talk to one another clearly. The first part of almost every technical discussion will be spent clarifying what words mean. This book will try to avoid the mess of terminology by adopting technical terms. A techni- cal term is not one that is technical, but rather a word for which a single meaning, out of the total range of possible meanings, has been chosen as “the” meaning. Each time you see the word switch, for instance, throughout this book, it will have a single, definite meaning. 4 Cisco Certified Support Technician CCST Networking 100-150 Official Cert Guide This approach makes it easier for me, the author, to communicate clearly to you, the reader—but the real world does not work this way. In the real world, once you are outside this book, you will need to remember that words in computer networking—even simple ones—might have a range of meanings. It’s important to be sensitive to this reality. Each time a word is defined in the text, two definitions will be given. The first will be the single (technical) meaning used throughout this book. A second part of the definition will describe a range of definitions used within the world of computer networking. Thinking About How to Think As you dive into this book, working toward this certification in computer networking, it’s important to know how to think about computer networks. Learning two basic concepts will help you on your journey toward understanding networks. Abstraction is a way of dealing with complexity. As a way of thinking, abstractions group things in some way—because they are alike, different, chronologically ordered, or something else. No one says, “I’ll meet you out in the area with thousands of grass blades, many kinds of flowers, and trees along the edges.” Everyone says, “I’ll meet you at the field.” Engineers must be able to see groups of things as abstractions, and then to break groups of things apart into their components—mentally—to deal with complexity. The problem and solution mindset is the second critical thinking skill engineers must have. Quickly finding the problem, understanding the kind of problem it is, knowing the broad set of solutions available for the problem, and then being able to pinpoint the best solution for the problem—these things are the hallmark of good engineering. Thinking About Change Technology is always changing. Networking technologies are not an exception. In some ways, change is good. There is always something interesting or exciting going on the world of technology. There are new inventions, new ideas, and new ways of doing things. Things are always getting faster, more efficient, or easier. On the other hand, constant change is bad. After a while, change itself becomes monotonous and boring. It’s hard to find a foothold on a constantly pitching deck. One way of countering the constant movement is to separate the fundamentals—the prob- lems and solutions, the ways of thinking—from their implementations in the real world. This book will give you the foundations you need to understand the basics, and to ask the right questions to build on those basics, achieving excellence as a network engineer. Laying the Foundation Computer networks are as vital to the modern world as any other kind of network—the transportation system, the supply chain, and the organization. Let’s begin by looking at the basic problems a computer network must solve to transport data between two devices: packaging things for shipment and finding the path to reach the destination.  Part 1: Foundations 5 The chapters in this part of the book are as follows: Chapter 1: Shipping Things Chapter 2: Addresses Chapter 3: Routing and Switching Chapter 4: Wired Host Networking Configuration Chapter 5: What’s in a Network? Chapter 6: Network Models CHAPTER 1 Shipping Things This chapter covers the following exam topics: 1. Standards and Concepts 1.1 Identify the fundamental building blocks of networks. TCP/IP model, OSI model, frames and packets, addressing 1.5 Describe common network applications and protocols. TCP vs. UDP (connection-oriented vs. connectionless), FTP, SFTP, TFTP, HTTP, HTTPS, DHCP, DNS, ICMP, NTP Let’s say you have something to ship to a friend—a necklace, a book (like this one), a poster, or a dog. Okay, maybe you don’t want to ship a dog, but the process is the same for just about everything else. You 1. Put the item in a box. 2. Seal the box. 3. Put an address on the box. 4. Take the box to a shipper (the post office, a commercial shipper, etc.). Shipping an item is a simple process from the outside, but simple and easy are not the same thing. Packages are delayed, damaged, and lost; someone must figure out how to track each package. Each address must be at least somewhat unique, and someone must know how to relate a person to a location. Someone must figure out how to move the package along well-known routes and what kind of transportation to use (boat, train, truck, airplane, or bicycle?). And all these things must happen at a hard-to-imagine scale. Millions of packages must be sealed, addressed, shipped, and received daily. Because the physical process of packaging data to carry it through a network is similar enough to shipping a physical item halfway across the world, it is helpful to start with mate- rial things. “Do I Know This Already?” Quiz Take the quiz (either here or use the PTP software) if you want to use the score to help you decide how much time to spend on this chapter. Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes,” found at the end of the book, includes both the answers and expla- nations. You can also find answers in the PTP testing software. Table 1-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping Section Questions The Purpose of Packaging 1, 2 Addressing 3, 4, 5 Packaging Data 6, 7, 8 Tunnels 9 CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chap- ter. If you do not know the answer to a question or are only partially sure of the answer, you should mark that question as wrong for purposes of the self-assessment. Giving yourself credit for an answer you incorrectly guess skews your self-assessment results and might pro- vide you with a false sense of security. 1. What is packaging a piece of data called to add an address and other metadata? a. Packet b. Encapsulation c. Labeling d. Packaging 2. What is metadata? a. Virtual data; used in computer networks to describe a virtual interface b. Information about Information; used in computer networking to describe the source, destination, and other information about a frame, packet, or segment c. Tunnel data; used in computer networks to describe the path a packet should take through the network d. Data that does not exist 3. What is a physical address? a. An address identifying a single physical interface b. An address identifying a single logical interface c. An address identifying a single host or device d. An address identifying a single application 4. What is an interface address? a. An address identifying a single physical interface b. An address identifying a single logical interface c. An address identifying a single host or device d. An address identifying a single application 5. What is a port? a. An address identifying a single physical interface b. An address identifying a single logical interface c. An address identifying a single host or device d. An address identifying a single application 8 Cisco Certified Support Technician CCST Networking 100-150 Official Cert Guide 6. What is a frame? a. Data packaged to be moved from one host to another b. Data packaged to be moved from one application to another c. Data packaged to be moved from one physical interface to another d. Data packaged to be moved from one application to a physical interface 7. What is a packet? a. Data packaged to be moved from one host to another b. Data packaged to be moved from one application to another c. Data packaged to be moved from one physical interface to another d. Data packaged to be moved from a host to a physical interface 8. What is a segment? a. Data packaged to be moved from one host to another b. Data packaged to be moved from one application to another c. Data packaged to be moved from one physical interface to another d. Data packaged to be moved from a host to an application 9. What does a tunnel hide in a computer network? a. The contents of the packet b. The source and destination physical interfaces c. The original destination addresses d. The original packet’s protocol Foundation Topics The Purpose of Packaging Packaging is the first step in shipping. Let’s look at packaging in the physical world and then consider how packaging in networking is similar and different. The Physical World Most people package things to protect them during shipment. We usually place an item in at least two packages—the commercial packaging, which tells us about the item, and an outer package protecting the inner packaging and the item itself. Packaging, however, is not limited to protecting things. A second purpose for packaging is to hide what we’re shipping. When you ship a present to a friend, you will place the commercial packaging inside another box, then wrap the box so your friend cannot see what’s inside until they rip the wrapping off. This wrapped package is placed inside a shipping box—so we now have three layers of packaging. Packages might also be placed inside packages to keep related items together. If you buy a computer, it is likely to come in one large box. When you open the large box, you find sepa- rate boxes for the keyboard, processing unit, etc. Finally, packages are placed inside other packages to make them easier to handle. Putting them all on a pallet is easier if you have several hundred packages to load into a container (for trans- port via ship, truck, or train). Moving one pallet is easier than moving a hundred separate boxes.  Chapter 1: Shipping Things 9 Figure 1-1 illustrates these layers of packaging. 1 Delivery Wrapped Truck Item Package Long-Distance Pallet Freight Commercial Package Shipping Box Figure 1-1 Physical Shipping Packages In Figure 1-1: 1. The manufacturer wraps the item in commercial packaging. 2. The shipper wraps the commercial packaging in gift wrap. 3. The shipper wraps the gift-wrapped package in a shipping box. 4. A driver picks up the shipping box and delivers it to a warehouse or shipping facility. 5. The shipping company adds the shipping box to a pallet with other shipping boxes addressed to recipients in the same geographic area. 6. The shipping company places the pallet on a flight, train, ship, or other transport. Each container is broken down at the other end until the recipient gets to the item. Computer Networks We tend to think of networks as being somehow “not connected to the real world.” We put a picture in over here, and it pops out over there—but data is still physical. Pictures are encoded in physical signals of some kind, the physical bits need to be moved to the receiver, and the receiver needs to display the image based on the bits received. We use different words to describe the packaging. Figure 1-2 illustrates the transmission process for computer networks. application-to-application A E host-to-host interface-to-interface B D C Figure 1-2 Packaging in Computer Networks 10 Cisco Certified Support Technician CCST Networking 100-150 Official Cert Guide As you do when shipping physical packages, you start with something to send—in this case, data. Application A wants to send data to application E. To do this, it puts the data in a pack- age, or encapsulates it, and hands it off to the host, B. Host B then puts the data into a second package and hands it off to a local network inter- face. The local interface then puts the data into a third package and transmits it across the physical link C toward the interface on host D. Once the interface on host D receives the data, it removes the outer package and hands off the inner package to the host (or the software within the host—the operating system). The host software then removes the outer package (once again) and sends the innermost package to application E. Application E then removes the innermost package and uses the data. The words used in this process are not the same as physical shipping; let’s go through a few. Encapsulating is putting data inside a new package. When you take an existing data pack- age and put it into another package—a larger box, in other words—you are encapsulating it. The box the data is put into is called encapsulation. Much like there are commercial packaging, shipping boxes, and pallets in physical shipping, each encapsulation has a name in computer networking: A frame is data encapsulated with the information needed to move from one physical interface to another physical interface. A packet is data encapsulated with the information needed to move from host to host. A segment is data encapsulated with the information needed to move from applica- tion to application. A segment may also be called a datagram. Sometimes you have several related items to send that will not fit into a single box. You can send them as a set of shipments, each somehow marked, so they are received and “pro- cessed” in the same order. In computer networking, applications often send information this way: an image or song will not fit into a single packet. It might take tens of thousands of packets to send a single piece of information or a set of interrelated pieces of information. Some protocols allow two applications to form a session to transfer information in streams, which supports breaking large pieces of data into smaller chunks for transmission or even collecting related information into a single group. Part III of this book, “Services,” will con- sider the idea of a transport service that carries data streams in more detail. Looking at Figure 1-2, you might notice what looks like layers—the applications communi- cate to one another across the top, the hosts communicate to one another in the middle, and the interfaces communicate to one another along the bottom. These are called just that— layers—in a computer network. These layers are the basis for models of computer network operation. Chapter 6, “Network Models,” will consider models more fully. NOTE While frame is rarely substituted for packet, packet is often substituted for frame or segment. Engineers often call data moving through the network packets regardless of the encapsulation or layer.  Chapter 1: Shipping Things 11 Addressing 1 Packaging also gives us a place to put the origin and destination addresses. The address tells the shipper to whom and where a package should be delivered. The Physical World It’s not so obvious the address on a package might change over the shipping process; Figure 1-3 illustrates. Delivery Wrapped Truck Item Package Long-Distance (no address) (recipient’s name) Pallet Freight (warehouse’s (city, airport code, etc.) Commercial physical address) Package Shipping Box (no address) (recipient’s physical address) Figure 1-3 Labels and Addresses Through the Shipping Process The item itself does not have an address. Because it is used by whoever happens to own or have physical possession of it, there is no real need for an address here (although you some- times see people put their names on toys, serving dishes, and other items). The commercial package, likewise, does not have an address. The wrapped gift, however, will normally have a name or some form of identification. It’s long been a tradition in our house to wrap Christmas presents so each person has their own “wrapping paper,” which eliminates the need for tags. However, this use of wrapping paper as an identifier is a good example of the importance of context when reading an address. There isn’t any need for any address other than a name when the package is destined for someone in the same house. The context of an address is its scope—the range in which the address makes sense without any other information. The scope of a gift tag is within a single household, office, or other small groups of people. Once the package has been placed in a shipping box, the recipient’s physical address is attached to the outside in some way. The recipient’s address is not always clear-cut; the address may include a “care of” indicating a business or caretaker. When we move from the gift tag to the shipping label, the scope of the address changes. The scope contains a lot more possible people and locations. Because of this, the label must have a lot more information. The scope changes once again when the shipping box is combined with many other boxes and placed on a pallet. The shipping company will address the pallet to a warehouse or ship- ping facility. Again, this address represents a physical location within the shipping network. 12 Cisco Certified Support Technician CCST Networking 100-150 Official Cert Guide Finally, the entire airplane itself acts as a shipping container. The destination, in this case, is not on the airplane but rather in the flight plan, and the destination is a city or airport. Moving from the gift tag to the shipping box increased the scope and the amount of infor- mation on the label simultaneously. More information is needed because the package no longer has the context of a small group of people. Moving from the shipping label to the pallet label, however, reduces the information on the label. Why is this? Because the person moving the pallet around does not want (or need) to know the shipping address of every box on the pallet. Knowing the destination address of every box on the pal- let would be confusing rather than helpful. Forcing the person moving the pallet to read all those addresses would also waste time and energy, so it would be inefficient. Addressing the pallet to a shipping facility rather than the destination of every box on the pallet is a form of abstraction; just the name of the city represents all the addresses within a city, a postal code, an airport code, or the location of a shipping warehouse. Abstraction is one of the most important concepts in network engineering. Computer Networks Data moving through a network also has a hierarchy of labels, much like physical shipping; Figure 1-4 illustrates. application address logical interface A address E physical interface address B D C Figure 1-4 Host Addresses For historical reasons, the names of the different addresses used in computer networks often overlap. A physical interface address was originally tied to an individual, physical computer interface. As computers became more powerful and new applications developed, the direct relationship between physical interface addresses and physical interfaces ended. First, virtual interfaces, which emulate physical interfaces, were invented. Virtual interfaces are like a pen or stage name; a single (physical) person has multiple identities, each of which is “used” in a different situation, and each of which can have a different “personality.” Aliases and multiple usernames across social media and gaming sites are another instance of the same kind of thing. A single person represents themselves using multiple identities, each of which might have slightly different characteristics.  Chapter 1: Shipping Things 13 A virtual interface may also be called a logical or software interface. While these three have slightly different meanings, in this book, all three will be called an interface or virtual 1 interface. Second, a single physical interface may also have multiple physical addresses—like an apart- ment building with multiple physical addresses within the same physical space or multiple businesses with offices in the same building. Physical interface addresses are Often shortened to physical address. Called Media Access Control (MAC) addresses. The Institute of Electrical and Electronics Engineers (IEEE) uses this term to describe the address of an interface that controls access to a physical transmission medium, such as a copper, optical, or wire- less link. Part II of this book discusses physical links in more detail. Called the Layer 2 address. The name “Layer 2” is taken from the second layer of the Open Systems Interconnection (OSI) model. Chapter 6 will consider the OSI model in more detail; for now, note the Layer 2 address is the address of a physical or virtual interface. NOTE I use the term physical address throughout this book to refer to these kinds of addresses. The logical interface address was originally tied to a logical interface. The network stack or network software, which is part of the host’s operating system, manages this logical interface. As with the physical address, the meaning and use of the logical interface address have expanded and changed. Logical interface addresses can identify a virtual interface, and a single interface may have multiple addresses (in fact, most logical interfaces have multiple addresses). Logical interface addresses are Often shortened to interface address. Called a host address. Some networking technologies identify an entire host with a single address (often called an identifier), and some devices have only a single interface. Called an Internet Protocol (IP) address. In most networks, the logical interface address will be an Internet Protocol (IP) address of some type, so this address is also called the IP address. There are many other kinds of logical interface addresses than IP. Called a Layer 3 address because these addresses are at the third layer of the OSI model. Chapter 6 describes the OSI model in detail. Because the third layer of the OSI model is called the network layer, these addresses are also sometimes called network addresses. The term network address has a dif- ferent meaning, described in a later chapter—terms in networking often have multiple meanings, which can be confusing. 14 Cisco Certified Support Technician CCST Networking 100-150 Official Cert Guide NOTE A logical interface address is called an interface address or IP address throughout this book. Applications have addresses too—addresses that only make sense within the context of an individual host. These addresses are called ports, sockets, or sometimes service identifiers. From the network’s perspective, the application is represented as a port or service identifier. The application is represented as a socket from the perspective of the software running on the host. NOTE Applications running on individual hosts are identified using ports throughout this book. Table 1-2 provides a quick reference to all the different names of addresses and how they are related. Table 1-2 Addresses Name Description Alternate Names Physical address A media access address Layer 2 address associated with a physical or MAC address virtual computer interface Interface address An address associated with a Layer 3 address logical software interface Logical interface address Host address IP address System ID or NET Port A communication channel to Socket an application running on a host Hosts and Devices Before we move more deeply into packaging, it is worth mentioning two other words used in computer networking with overlapping meanings: host and device. When computers were large enough to be shared resources, the host “hosted,” or ran, soft- ware, while the terminal allowed users to access the computer. Devices, on the other hand, were things like printers and keyboards. When desktop computers (called microcomputers at the time) became commonplace, they were called hosts for the same reason: they hosted (ran) software applications. Embedding computers in more (and smaller) devices blurs the line between the host and device. No clear line between the two kinds of devices any longer exists. I will use the term host throughout this book to describe what most people consider a stan- dard computer, like a desktop or laptop. Device describes anything connected to a network, like printers, tablets, doorbells, and toasters.  Chapter 1: Shipping Things 15 Packaging Data 1 Physical packaging is intuitive; cardboard boxes, plastic bags, bubble bags, envelopes, pallets, and crates are all common. What about data? You cannot put a digital image in a cardboard box and push it through a copper wire, so how do you package data? What does encapsulation do in a computer network, and what does it look like? Encapsulation is putting a header in front of the data describing The source of the data, the source address The destination, the destination address The service this data needs to be passed to when it reaches its destination to be processed Any other information needed for the network to forward the data from the source to the destination Information about the packet is always added to the front of the packet, so it is carried in a packet header. A new header is added with the correct information to encapsulate a packet, as shown in Figure 1-5. 1 data A SP DP data 2 3 SA DA SP DP data 4 SA DA SA DA SP DP data B C D 5 SA DA SA DA SP DP data 6 SA DA SP DP data 7 SP DP data 8 data E Figure 1-5 Encapsulation Process Each step in the process is shown as a number on the illustration: 1. Application A creates some data it would like to send to application E. The data might be an image, text message, emoji, or any kind of digital data. 2. Application A encapsulates the data in a header containing (at least) source and destination ports (SP and DP in the illustration). 16 Cisco Certified Support Technician CCST Networking 100-150 Official Cert Guide 3. Host B’s network stack will determine the correct interface address to send the data. Host B’s network stack will determine the correct destination is an interface address on host D and add a new header on the packet with the correct source and destination addresses (SA and DA in the illustration). Host B will also add a protocol number indi- cating which process on host D should process the information. The protocol number is just like a socket, but it is an address for a network stack rather than an application. 4. Host B’s network stack will send the newly encapsulated data to the physical interface through which host D can be reached. The physical interface will add another header containing host B’s physical address as the source and host D’s address as the destina- tion (SA and DA in the illustration). At this point, the data is contained inside a packet, which is then contained inside a frame. 5. When host D receives the frame, the physical interface software will recognize “this frame is for me” by examining the outermost header’s destination address. After deter- mining whether it should process the packet, host D’s physical interface software will use information in the outer header to determine which process should process the information contained in the frame using the protocol number. Once the physical interface software at host D determines where it should send the packet on the local host, it strips the outer header, de-encapsulating the frame. 6. The network stack at host D receives the packet and uses the socket number to deter- mine which application running on the local host should receive this information. Once it has determined the correct application, the network stack strips the outer header, de-encapsulating the packet, and leaving just the application’s header on the data. 7. Application E receives the packet, examines the remaining header for useful information, then strips this final header and processes the data. One additional encapsulation layer—the transport protocol—has been left out of this description for clarity (step 8 in Figure 1-5). Chapter 14, “Network Transport,” examines transport protocols in more detail. What Is a Protocol? If you were to meet the queen or president of some country, you would need to follow a protocol—when to speak, when to bow, when to curtsy, when to leave, etc. When you read a book, you read from left to right in some languages and from right to left in others. You introduce yourself when you attend a gathering the first time, such as a church or bingo game. All of these are protocols. A protocol is nothing more than a way of doing things. Communications protocols have a dictionary—what symbols mean—and a grammar—the proper ordering of elements like nouns and verbs, or even the way the context of a symbol changes its meaning. Computers are communication networks, so they have protocols. Network protocols, for instance, determine how 0s and 1s translate into numbers, letters, and actions. All the protocols in computer networks fall into one of two categories—transport and routing—both of which are covered in more detail later in this book.  Chapter 1: Shipping Things 17 NOTE Transport protocols are also sometimes called routed protocols, which should not 1 be confused with a routing protocol. This book uses the term transport protocol rather than routed protocol, because not all transport protocols are routed, and routable transport protocols are not always routed. What Is Metadata? Data is straightforward: it is just knowledge about something. For instance, if you know someone’s favorite color, music group, or car, you have data about them. Metadata is just as easy to define: metadata is data about data (or information about information). Defining something is not the same thing as making sense of (or understanding) it. Perhaps a real-world example will help. The batting average of a baseball team and the scoring average of a football team are both data—information about the team. The trend of the batting and scoring averages can be considered metadata. While the trend is information about the team, it is also about the bat- ting and scoring averages. In many situations, classifying a piece of information as either data or metadata is a matter of perspective. The difference between data and metadata is more apparent in building packets to carry through a computer network. An address is data when used to describe an interface or host. An address is metadata when used to describe where a piece of data needs to be delivered. The delivery address is something you know about the data; hence, it is metadata. Tunnels Tunnels are one of the most challenging concepts to understand in computer networks because there are few real-world examples and because so many things seem to be tunnels—but they are not. The word tunnel evokes a tunnel through a mountain or under a body of water—like the Lincoln Tunnel crossing under the Hudson River between New Jersey and New York. Tunnels in computer networks do not go under something else; instead, they usually go over some- thing else. The sense we can bring from real-world tunnels to a computer network is they hide things. If you are sitting on a boat on the Hudson River directly above the Lincoln Tunnel, you would not have any idea there are hundreds of people driving along under you. In the same way, a computer network tunnel hides packets being carried through the net- work from the underlying network by encapsulating it. Specifically, a tunnel hides the origi- nal source and destination from the network over which the tunnel passes. Another way of understanding a tunnel is through the head-end and tail-end. The head-end of a tunnel is where the encapsulation is added, and the tail-end is where the encapsulation is removed. 18 Cisco Certified Support Technician CCST Networking 100-150 Official Cert Guide This description makes it sound as though the encapsulation of a packet into a frame is a form of tunneling, though, and it is not. Why not? Because tunnels go over in a network rather than under. Physical interfaces are logically under logical interfaces; encapsulating packets into frames so one physical interface can transmit the data to another physical inter- face is under. Figure 1-6 illustrates tunneling. A 1 APP HDR data virtual interface 2 VI HDR APP HDR data 3 INT HDR VI HDR APP HDR data 4 PHY HDR INT HDR VI HDR APP HDR data B C D E Figure 1-6 A High-Level Overview of Tunneling In Figure 1-6, 1. The application, A, encapsulates some data (APP HDR in the illustration) and sends it to a local interface. 2. The interface application A sends the data to is virtual. From application A’s perspec- tive, there is no difference between the physical and virtual interfaces. The virtual interface processes the packet the same way any other interface would, encapsulating the packet into a new header (VI HDR in the illustration). This VI HDR hides the origi- nal destination from lower layers by adding a second set of addresses to the packet. 3. Host B’s network stack treats this packet like any other, processing it and sending it to the destination address provided in the outermost header. In this case, the outermost address will be a tunnel endpoint rather than host D’s interface address. 4. Host B’s physical interface software will (finally) encapsulate the packet with a header containing host D’s physical address. The critical point here is not the kind of header used by the virtual interface to encapsulate the packet but rather the destination address. If the destination address is a type of interface that would normally be below the virtual interface, and the destination must de-encapsulate the packet before discovering its “true destination,” the packet is being tunneled.  Chapter 1: Shipping Things 19 Later chapters will discuss tunneling in more detail. The critical point to remember here is the protocol used is not what determines whether a packet is tunneled. The destination of 1 the packet and whether the final destination is being hidden from underlying network ele- ments determine whether a packet is being tunneled. Chapter Review This chapter used physical world examples to explain the importance and uses of packaging—or encapsulation—and addressing in computer networks. You should also have learned about what a protocol is, what metadata is, and what a tunnel is. One point to remember: addresses in computer networks can refer to many things, including a physical interface, a logical interface, a host (device), or even an application. Clearly differentiating these different kinds of addresses will help you lay a solid foundation for understanding computer networks. Now that you understand the purpose of addresses, the next chapter will discuss how to work with addresses in computer networks. One key to doing well on the exams is to perform repetitive spaced review sessions. Review this chapter’s material using either the tools in the book or interactive tools for the same material found on the book’s companion website. Refer to the online Appendix D, “Study Planner,” element for more details. Table 1-3 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column. Table 1-3 Chapter Review Tracking Review Element Review Date (s) Resource Used Review key topics Book, website Review key terms Book, website Repeat DIKTA questions Book, PTP Review concepts and actions Book, website Review All the Key Topics Table 1-4 lists the key topics for this chapter. Table 1-4 Key Topics for Chapter 1 Key Topic Element Description Page Number Paragraph Encapsulating and 10 encapsulation List Distinction between a frame, 10 packet, and segment Paragraph Sessions and streams 10 Paragraph Physical interface addresses 12 Paragraph Virtual interfaces 12 20 Cisco Certified Support Technician CCST Networking 100-150 Official Cert Guide Key Topic Element Description Page Number Paragraph Logical interface addresses 13 Paragraph Application addresses: ports, 14 sockets, service identifiers Figure 1-5 Encapsulation process 15 Paragraph Protocol definition 16 Paragraph Metadata definition 17 Paragraph Computer network tunnel 17 definition Figure 1-6 Tunneling process 18 Key Terms You Should Know Key terms in this chapter include encapsulation, frame, packet, segment, physical interface address, physical address, logical interface address, interface address, port, socket, protocol, metadata, tunnel Concepts and Actions Review the concepts considered in this chapter using Table 1-5. You can cover the right side of this table and describe each concept or action in your own words to verify your understanding. Table 1-5 Concepts and Actions Encapsulate Adding a header to data to carry metadata such as the source and destination addresses Frame Data encapsulated to be carried from interface to interface Packet Data encapsulated to be carried from host to host Segment Data encapsulated to be carried from application to application Address context The scope in which the address uniquely describes a receiver or set of receivers Physical interface A physical point of connection where electrical or optical signals are converted to and from data Virtual interface A logical point of connection that acts like a physical interface; no physical hardware is associated with a virtual interface Media Access Official IEEE name for a physical interface address Control Logical interface A higher-layer interface defined in software and related to a physical or virtual interface Port The address of an individual application running on a host Host A device that can host or run applications Header contents At a minimum, the source address, destination address, and some way to indicate a service  Chapter 1: Shipping Things 21 Protocol A way of doing things that enables communications 1 Metadata Data about a collection of data Tunnel Hiding packet contents and headers from intermediate devices by adding a layer of encapsulation Head-end Where packets are encapsulated to be carried through a tunnel Tail-end Where packets are de-encapsulated and removed from a tunnel