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The TCP/IP and OSI Networking Models

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Chapter 1. The TCP/IP and OSI Networking Models
This chapter covers the following exam topics:
Operation of IP Data Networks
Recognize the purpose and functions of various network devices such as Routers, Switches, Bridges and Hubs.
Select the components required to meet a given network specification. Describe the purpose and basic operation of the protocols in the OSI and TCP/IP models. Welcome to the first chapter in your study for CCENT and CCNA! This chapter begins Part I, which focuses on the basics of networking. Because networks require all the devices to follow the rules, this part starts with a discussion of networking models, which gives you a big-picture view of the networking rules. You can think of a networking model as you think of a set of architectural plans for building a house. A lot of different people work on building your house, such as framers, electricians, bricklayers, painters, and so on. The blueprint helps ensure that all the different pieces of the house work together as a whole. Similarly, the people who make networking products, and the people who use those products to build their own computer networks, follow a particular networking model. That networking model defines rules about how each part of the network should work, as well as how the parts should work together, so that the entire network functions correctly. The CCNA exams include detailed coverage of one networking model: Transmission Control Protocol/Internet Protocol (TCP/IP). TCP/IP is the most pervasively used networking model in the history of networking. You can find support for TCP/IP on practically every computer operating system (OS) in existence today, from mobile phones to mainframe computers. Every network built using Cisco products today supports TCP/IP. And not surprisingly, the CCNA exams focus heavily on TCP/IP.

The ICND1 exam, and the ICND2 exam to a small extent, also covers a second networking model, called the Open Systems Interconnection (OSI) reference model. Historically, OSI was the first large effort to create a vendor-neutral networking model. Because of that timing, many of the terms used in networking today come from the OSI model, so this chapter’s section on OSI discusses OSI and the related terminology.

“Do I Know This Already?†Quiz
Use the “Do I Know This Already?†quiz to help decide whether you might want to skim this chapter, or a major section, moving more quickly to the “Exam Preparation Tasks†section near the end of the chapter. You can find the answers at the bottom of the page following the quiz. For thorough explanations, see DVD Appendix C, “Answers to the â€‫ک‬Do I Know This Already?’ Quizzes.â€‌ Table 1-1. Do I Know This Already?†Foundation Topics Section-to-Question Mapping

1. Which of the following protocols are examples of TCP/IP transport layer protocols? (Choose two answers.) a. Ethernet
b. HTTP
c. IP
d. UDP
e. SMTP

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f. TCP
2. Which of the following protocols are examples of TCP/IP data link layer protocols? (Choose two answers.) a. Ethernet
b. HTTP
c. IP
d. UDP
e. SMTP
f. TCP
g. PPP
3. The process of HTTP asking TCP to send some data and making sure that it is received correctly is an example of what?
a. Same-layer interaction
b. Adjacent-layer interaction
c. OSI model
d. All of these answers are correct.
4. The process of TCP on one computer marking a TCP segment as segment 1, and the receiving computer then acknowledging the receipt of TCP segment 1 is an example of what? a. Data encapsulation
b. Same-layer interaction
c. Adjacent-layer interaction
d. OSI model
e. All of these answers are correct.
5. The process of a web server adding a TCP header to the contents of a web page, followed by adding an IP header and then adding a data link header and trailer is an example of what? a. Data encapsulation

b. Same-layer interaction
c. OSI model
d. All of these answers are correct.
6. Which of the following terms is used specifically to identify the entity created when encapsulating data inside data link layer headers and trailers?
a. Data
b. Chunk
c. Segment
d. Frame
e. Packet
7. Which OSI layer defines the functions of logical network-wide addressing and routing? a. Layer 1
b. Layer 2
c. Layer 3
d. Layer 4
e. Layer 5, 6, or 7
8. Which OSI layer defines the standards for cabling and connectors? a. Layer 1
b. Layer 2
c. Layer 3

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d. Layer 4
e. Layer 5, 6, or 7
9. Which of the following terms are not valid terms for the names of the seven OSI layers? (Choose two answers.)
a. Application
b. Data link
c. Transmission
d. Presentation
e. Internet
f. Session
Answers to the “Do I Know This Already?†quiz:
1 D and F 2 A and G 3 B 4 B 5 A 6 D 7 C 8 A 9 C and E

Foundation Topics
This chapter introduces some of the most basic ideas about computer networking, while also defining the structure of two networking models: TCP/IP and OSI. The chapter begins with a brief introduction of how most people view a network, which hopefully connects with where you are to start your CCNA journey. The middle of this chapter introduces networking by explaining some of the key features of TCP/IP. The chapter closes with some additional concepts and terminology related to the OSI model. Perspectives on Networking

So, you are new to networking. Like many people, your perspective about networks might be that of a user of the network, as opposed to the network engineer who builds networks. For some, your view of networking might be based on how you use the Internet, from home, using a high-speed Internet connection like DSL or cable TV, as shown in Figure 1-1.

Figure 1-1. End-User Perspective on High-Speed Internet Connections The top part of the figure shows a typical high-speed cable Internet user. The PC connects to a cable modem using an Ethernet cable. The cable modem then connects to a cable TV (CATV) outlet in the wall using a round coaxial cable—the same kind of cable used to connect your TV to the CATV wall outlet. Because cable Internet services provide service continuously, the user can just sit down at the PC and start sending e-mail, browsing websites, making Internet phone calls, and using other tools and applications as well. The lower part of the figure uses two different technologies. First, the tablet computer uses wireless technology that goes by the name wireless local-area network (wireless LAN), or Wi-Fi, instead of using an Ethernet cable. In this example, the router uses a different technology, digital subscriber line (DSL), to communicate with the Internet.

The CCNA exams, and particularly the ICND1 (100-101) exam, include the technologies used to create networks at a home (as shown in Figure 1-1), but with even more focus on networking technology used inside a company. The Information Technology (IT) world refers to a network created by one corporation, or enterprise, for the

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purpose of allowing its employees to communicate, as an enterprise network. The smaller networks at home, when used for business purposes, often go by the name small office home office (SOHO) networks. Users of enterprise networks have some idea about the enterprise network at their company or school. People realize that they use a network for many tasks. PC users might realize that their PC connects through an Ethernet cable to a matching wall outlet, as shown at the top of Figure 1-2. Those same users might use wireless LANs with their laptop when going to a meeting in the conference room as well. Figure 1-2 shows these two end-user perspectives on an enterprise network.

Figure 1-2. Example Representation of an Enterprise Network
Note
In networking diagrams, a cloud represents a part of a network whose details are not important to the purpose of the diagram. In this case, Figure 1-2 ignores the details of how to create an Enterprise network.

Some users might not even have a concept of the network at all. Instead, these users just enjoy the functions of the network—the ability to post messages to social media sites, make phone calls, search for information on the Internet, listen to music, and download countless apps to their phones—without caring about how it works or how their favorite device connects to the network.

Regardless of how much you already know about how networks work, this book, and the related certifications, help you learn how networks do their job. That job is simply this: moving data from one device to another. The rest of this chapter, and the rest of this first part of the book, reveals the basics of how to build both SOHO and enterprise networks so that they can deliver data between two devices. In the building business, much work happens before you nail the first boards together. The process starts with some planning, an understanding of how to build a house, and some architectural blueprints of how to build that specific house. Similarly, the journey toward building any computer network does not begin by installing devices and cables, but instead by looking at the architectural plans for those modern networks: the TCP/IP model. TCP/IP Networking Model

A networking model, sometimes also called either a networking architecture or networking blueprint, refers to a comprehensive set of documents. Individually, each document describes one small function required for a network; collectively, these documents define everything that should happen for a computer network to work. Some documents define a protocol, which is a set of logical rules that devices must follow to communicate. Other documents define some physical requirements for networking. For example, a document could define the voltage and current levels used on a particular cable when transmitting data. You can think of a networking model as you think of an architectural blueprint for building a house. Sure, you can build a house without the blueprint. However, the blueprint can ensure that the house has the right foundation and structure so that it will not fall down, and it has the correct hidden spaces to accommodate the plumbing, electrical, gas, and so on. Also, the many different people that build the house using the blueprint—such as framers, electricians, bricklayers, painters, and so on—know that if they follow the blueprint, their part of the work should not cause problems for the other workers.

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Similarly, you could build your own network—write your own software, build your own networking cards, and so on—to create a network. However, it is much easier to simply buy and use products that already conform to some well-known networking model or blueprint. Because the networking product vendors build their products with some networking model in mind, their products should work well together. History Leading to TCP/IP

Today, the world of computer networking uses one networking model: TCP/IP (Transmission Control Protocol/Internet Protocol). However, the world has not always been so simple. Once upon a time, networking protocols didn’t exist, including TCP/IP. Vendors created the first networking protocols; these protocols supported only that vendor’s computers. For example, IBM published its Systems Network Architecture (SNA) networking model in 1974. Other vendors also created their own proprietary networking models. As a result, if your company bought computers from three vendors, network engineers often had to create three different networks based on the networking models created by each company, and then somehow connect those networks, making the combined networks much more complex. The left side of Figure 1-3 shows the general idea of what a company’s enterprise network might have looked back in the 1980s, before TCP/IP became common in enterprise internetworks.

Figure 1-3. Historical Progression: Proprietary Models to the Open TCP/IP Model Although vendor-defined proprietary networking models often worked well, having an open, vendor-neutral networking model would aid competition and reduce complexity. The International Organization for Standardization (ISO) took on the task to create such a model, starting as early as the late 1970s, beginning work on what would become known as the Open Systems Interconnection (OSI) networking model. ISO had a noble goal for the OSI model: to standardize data networking protocols to allow communication between all computers across the entire planet. ISO worked toward this ambitious and noble goal, with participants from most of the technologically developed nations on Earth participating in the process. A second, less formal effort to create an open, vendor-neutral, public networking model sprouted forth from a U.S. Department of Defense (DoD) contract. Researchers at various universities volunteered to help further develop the protocols surrounding the original DoD work. These efforts resulted in a competing open networking model called TCP/IP.

During the 1990s, companies began adding OSI, TCP/IP, or both to their enterprise networks. However, by the end of the 1990s, TCP/IP had become the common choice, and OSI fell away. The center part of Figure 1-3 shows the general idea behind enterprise networks in that decade—still with networks built upon multiple networking models, but including TCP/IP.

Here in the twenty-first century, TCP/IP dominates. Proprietary networking models still exist, but they have mostly been discarded in favor of TCP/IP. The OSI model, whose development suffered in part because of a slower formal standardization process as compared with TCP/IP, never succeeded in the marketplace. And TCP/IP, the networking model originally created almost entirely by a bunch of volunteers, has become the most

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prolific network model ever, as shown on the right side of Figure 1-3. In this chapter, you will read about some of the basics of TCP/IP. Although you will learn some interesting facts about TCP/IP, the true goal of this chapter is to help you understand what a networking model or networking architecture really is and how it works.

Also in this chapter, you will learn about some of the jargon used with OSI. Will any of you ever work on a computer that is using the full OSI protocols instead of TCP/IP? Probably not. However, you will often use terms relating to OSI. Also, the ICND1 exam covers the basics of OSI, so this chapter also covers OSI to prepare you for questions about it on the exam.

Overview of the TCP/IP Networking Model
The TCP/IP model both defines and references a large collection of protocols that allow computers to communicate. To define a protocol, TCP/IP uses documents called Requests for Comments (RFC). (You can find these RFCs using any online search engine.) The TCP/IP model also avoids repeating work already done by some other standards body or vendor consortium by simply referring to standards or protocols created by those groups. For example, the Institute of Electrical and Electronic Engineers (IEEE) defines Ethernet LANs; the TCP/IP model does not define Ethernet in RFCs, but refers to IEEE Ethernet as an option. An easy comparison can be made between telephones and computers that use TCP/IP. You go to the store and buy a phone from one of a dozen different vendors. When you get home and plug in the phone to the same cable in which your old phone was connected, the new phone works. The phone vendors know the standards for phones in their country and build their phones to match those standards. Similarly, when you buy a new computer today, it implements the TCP/IP model to the point that you can usually take the computer out of the box, plug in all the right cables, turn it on, and it connects to the network. You can use a web browser to connect to your favorite website. How? Well, the OS on the computer implements parts of the TCP/IP model. The Ethernet card, or wireless LAN card, built into the computer implements some LAN standards referenced by the TCP/IP model. In short, the vendors that created the hardware and software implemented TCP/IP.

To help people understand a networking model, each model breaks the functions into a small number of categories called layers. Each layer includes protocols and standards that relate to that category of functions. TCP/IP actually has two alternative models, as shown in Figure 1-4.

Figure 1-4. Two TCP/IP Networking Models
The model on the left shows the original TCP/IP model listed in RFC 1122, which breaks TCP/IP into four layers. The top two layers focus more on the applications that need to send and receive data. The bottom layer focuses on how to transmit bits over each individual link, with the internet layer focusing on delivering data over the entire path from the original sending computer to the final destination computer. The TCP/IP model on the right is a common method used today to refer to the layers formed by expanding the original model’s link layer on the left into two separate layers: data link and physical (similar to the lower two layers of the OSI model). Note that the model on the right is used more often today.

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Note
The original TCP/IP model’s link layer has also been referred to as the network access and network interface layer.
Many of you will have already heard of several TCP/IP protocols, like the examples listed in Table 1-2. Most of the protocols and standards in this table will be explained in more detail as you work through this book. Following the table, this section takes a closer look at the layers of the TCP/IP model. Table 1-2. TCP/IP Architectural Model and Example Protocols

TCP/IP Application Layer
TCP/IP application layer protocols provide services to the application software running on a computer. The application layer does not define the application itself, but it defines services that applications need. For example, application protocol HTTP defines how web browsers can pull the contents of a web page from a web server. In short, the application layer provides an interface between software running on a computer and the network itself. Arguably, the most popular TCP/IP application today is the web browser. Many major software vendors either have already changed or are changing their application software to support access from a web browser. And thankfully, using a web browser is easy: You start a web browser on your computer and select a website by typing the name of the website, and the web page appears.

HTTP Overview

What really happens to allow that web page to appear on your web browser? Imagine that Bob opens his browser. His browser has been configured to automatically ask for web server Larry’s default web page, or home page. The general logic looks like Figure 1-5.

Figure 1-5. Basic Application Logic to Get a Web Page
So, what really happened? Bob’s initial request actually asks Larry to send his home page back to Bob. Larry’s web server software has been configured to know that the default web page is contained in a file called home.htm. Bob receives the file from Larry and displays the contents of the file in Bob’s web-browser window.

HTTP Protocol Mechanisms

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Taking a closer look, this example shows how applications on each endpoint computer—specifically, the webbrowser application and web-server application—use a TCP/IP application layer protocol. To make the request for a web page and return the contents of the web page, the applications use the Hypertext Transfer Protocol (HTTP).

HTTP did not exist until Tim Berners-Lee created the first web browser and web server in the early 1990s. Berners-Lee gave HTTP functions to ask for the contents of web pages, specifically by giving the web browser the ability to request files from the server, and giving the server a way to return the content of those files. The overall logic matches what was shown in Figure 1-5; Figure 1-6 shows the same idea, but with details specific to HTTP.

Figure 1-6. HTTP Get Request, HTTP Reply, and One Data-Only Message Note
The full version of most web addresses—also called Uniform Resource Locators (URL) —begins with the letters “http,†which means that HTTP is used to transfer the web pages.
To get the web page from Larry, at Step 1, Bob sends a message with an HTTP header. Generally, protocols use headers as a place to put information used by that protocol. This HTTP header includes the request to “get†a file. The request typically contains the name of the file (home.htm, in this case), or if no filename is mentioned, the web server assumes that Bob wants the default web page.

Step 2 in Figure 1-6 shows the response from web server Larry. The message begins with an HTTP header, with a return code (200), which means something as simple as “OK†returned in the header. HTTP also defines other return codes so that the server can tell the browser whether the request worked. (Here is another example: If you ever looked for a web page that was not found, and then received an HTTP 404 “not found†error, you received an HTTP return code of 404.) The second message also includes the first part of the requested file. Step 3 in Figure 1-6 shows another message from web server Larry to web browser Bob, but this time without an HTTP header. HTTP transfers the data by sending multiple messages, each with a part of the file. Rather than wasting space by sending repeated HTTP headers that list the same information, these additional messages simply omit the header.

TCP/IP Transport Layer
Although many TCP/IP application layer protocols exist, the TCP/IP transport layer includes a smaller number of protocols. The two most commonly used transport layer protocols are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

Transport layer protocols provide services to the application layer protocols that reside one layer higher in the TCP/IP model. How does a transport layer protocol provide a service to a higher-layer protocol? This section introduces that general concept by focusing on a single service provided by TCP: error recovery. Later chapters examine the transport layer in more detail, and discuss more functions of the transport layer. TCP Error Recovery Basics

To appreciate what the transport layer protocols do, you must think about the layer above the transport layer, the application layer. Why? Well, each layer provides a service to the layer above it, like the error-recovery service

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provided to application layer protocols by TCP.
For example, in Figure 1-5, Bob and Larry used HTTP to transfer the home page from web server Larry to Bob’s web browser. But what would have happened if Bob’s HTTP GET request had been lost in transit through the TCP/IP network? Or, what would have happened if Larry’s response, which included the contents of the home page, had been lost? Well, as you might expect, in either case, the page would not have shown up in Bob’s browser.

TCP/IP needs a mechanism to guarantee delivery of data across a network. Because many application layer protocols probably want a way to guarantee delivery of data across a network, the creators of TCP included an error-recovery feature. To recover from errors, TCP uses the concept of acknowledgments. Figure 1-7 outlines the basic idea behind how TCP notices lost data and asks the sender to try again.

Figure 1-7. TCP Error-Recovery Services as Provided to HTTP
Figure 1-7 shows web server Larry sending a web page to web browser Bob, using three separate messages. Note that this figure shows the same HTTP headers as Figure 1-6, but it also shows a TCP header. The TCP header shows a sequence number (SEQ) with each message. In this example, the network has a problem, and the network fails to deliver the TCP message (called a segment) with sequence number 2. When Bob receives messages with sequence numbers 1 and 3, but does not receive a message with sequence number 2, Bob realizes that message 2 was lost. That realization by Bob’s TCP logic causes Bob to send a TCP segment back to Larry, asking Larry to send message 2 again.

Same-Layer and Adjacent-Layer Interactions

The example in Figure 1-7 also demonstrates a function called adjacent-layer interaction, which refers to the concepts of how adjacent layers in a networking model, on the same computer, work together. In this example, the higher-layer protocol (HTTP) wants error recovery, and the higher layer uses the next lower-layer protocol (TCP) to perform the service of error recovery; the lower layer provides a service to the layer above it. Figure 1-7 also shows an example of a similar function called same-layer interaction. When a particular layer on one computer wants to communicate with the same layer on another computer, the two computers use headers to hold the information that they want to communicate. For example, in Figure 1-7, Larry set the sequence numbers to 1, 2, and 3 so that Bob could notice when some of the data did not arrive. Larry’s TCP process created that TCP header with the sequence number; Bob’s TCP process received and reacted to the TCP segments. This process through which two computers set and interpret the information in the header used by that layer is called same-layer interaction, and it occurs between computers that are communicating over a network. Table 1-3 summarizes the key points about how adjacent layers work together on a single computer and how one layer on one computer works with the same networking layer on another computer.

Table 1-3. Summary: Same-Layer and Adjacent-Layer Interactions

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TCP/IP Network Layer
The application layer includes many protocols. The transport layer includes fewer, most notably, TCP and UDP. The TCP/IP network layer includes a small number of protocols, but only one major protocol: the Internet Protocol (IP). In fact, the name TCP/IP is simply the names of the two most common protocols (TCP and IP) separated by a /.

IP provides several features, most importantly, addressing and routing. This section begins by comparing IP’s addressing and routing with another commonly known system that uses addressing and routing: the postal service. Following that, this section introduces IP addressing and routing. (More details follow in Chapter 4, “Fundamentals of IPv4 Addressing and Routing.â€)

Internet Protocol and the Postal Service

Imagine that you just wrote two letters: one to a friend on the other side of the country and one to a friend on the other side of town. You addressed the envelopes and put on the stamps, so both are ready to give to the postal service. Is there much difference in how you treat each letter? Not really. Typically, you would just put them in the same mailbox and expect the postal service to deliver both letters. The postal service, however, must think about each letter separately, and then make a decision of where to send each letter so that it is delivered. For the letter sent across town, the people in the local post office probably just need to put the letter on another truck.

For the letter that needs to go across the country, the postal service sends the letter to another post office, then another, and so on, until the letter gets delivered across the country. At each post office, the postal service must process the letter and choose where to send it next.

To make it all work, the postal service has regular routes for small trucks, large trucks, planes, boats, and so on, to move letters between postal service sites. The service must be able to receive and forward the letters, and it must make good decisions about where to send each letter next, as shown in Figure 1-8.

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Figure 1-8. Postal Service Forwarding (Routing) Letters
Still thinking about the postal service, consider the difference between the person sending the letter and the work that the postal service does. The person sending the letters expects that the postal service will deliver the letter most of the time. However, the person sending the letter does not need to know the details of exactly what path the letters take. In contrast, the postal service does not create the letter, but they accept the letter from the customer. Then, the postal service must know the details about addresses and postal codes that group addresses into larger groups, and it must have the ability to deliver the letters. The TCP/IP application and transport layers act like the person sending letters through the postal service. These upper layers work the same way regardless of whether the endpoint host computers are on the same LAN or are separated by the entire Internet. To send a message, these upper layers ask the layer below them, the network layer, to deliver the message.

The lower layers of the TCP/IP model act more like the postal service to deliver those messages to the correct destinations. To do so, these lower layers must understand the underlying physical network because they must choose how to best deliver the data from one host to another. So, what does this all matter to networking? Well, the network layer of the TCP/IP networking model, primarily defined by the Internet Protocol (IP), works much like the postal service. IP defines that each host computer should have a different IP address, just as the postal service defines addressing that allows unique addresses for each house, apartment, and business. Similarly, IP defines the process of routing so that devices called routers can work like the post office, forwarding packets of data so that they are delivered to the correct destinations. Just as the postal service created the necessary infrastructure to deliver letters—post offices, sorting machines, trucks, planes, and personnel—the network layer defines the details of how a network infrastructure should be created so that the network can deliver data to all computers in the network. Note

TCP/IP defines two versions of IP: IP Version 4 (IPv4) and IP Version 6 (IPv6). The world still mostly uses IPv4, so this introductory part of the book uses IPv4 for all references to IP. Later in this book, Part VII, “IP Version 6,†discusses this newer version of the IP protocol.

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Internet Protocol Addressing Basics

IP defines addresses for several important reasons. First, each device that uses TCP/IP—each TCP/IP host—needs a unique address so that it can be identified in the network. IP also defines how to group addresses together, just like the postal system groups addresses based on postal codes (like ZIP codes in the United States). To understand the basics, examine Figure 1-9, which shows the familiar web server Larry and web browser Bob; but now, instead of ignoring the network between these two computers, part of the network infrastructure is included.

Figure 1-9. Simple TCP/IP Network: Three Routers with IP Addresses Grouped First, note that Figure 1-9 shows some sample IP addresses. Each IP address has four numbers, separated by periods. In this case, Larry uses IP address 1.1.1.1, and Bob uses 2.2.2.2. This style of number is called a dotteddecimal notation (DDN). Figure 1-9 also shows three groups of address. In this example, all IP address that begin with 1 must be on the upper left, as shown in shorthand in the figure as 1._._._. All addresses that begin with 2 must be on the right, as shown in shorthand as 2._._._. Finally, all IP addresses that begin with 3 must be at the bottom of the figure. Additionally, Figure 1-9 also introduces icons that represent IP routers. Routers are networking devices that connect the parts of the TCP/IP network together for the purpose of routing (forwarding) IP packets to the correct destination. Routers do the equivalent of the work done by each post office site: They receive IP packets on various physical interfaces, make decisions based on the IP address included with the packet, and then physically forward the packet out some other network interface.

IP Routing Basics

The TCP/IP network layer, using the IP protocol, provides a service of forwarding IP packets from one device to another. Any device with an IP address can connect to the TCP/IP network and send packets. This section shows a basic IP routing example for perspective.

Note
The term IP host refers to any device, regardless of size or power, that has an IP address and connects to any TCP/IP network.
Figure 1-10 repeats the familiar case in which web server Larry wants to send part of a web page to Bob, but now

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with details related to IP. On the lower left, note that server Larry has the familiar application data, HTTP header, and TCP header ready to send. Additionally, the message now also contains an IP header. The IP header includes a source IP address of Larry’s IP address (1.1.1.1) and a destination IP address of Bob’s IP address (2.2.2.2).

Figure 1-10. Basic Routing Example
Step 1, on the left of Figure 1-10, begins with Larry being ready to send an IP packet. Larry’s IP process chooses to send the packet to some router—a nearby router on the same LAN—with the expectation that the router will know how to forward the packet. (This logic is much like you or me sending all our letters by putting them in a nearby post office box.) Larry doesn’t need to know anything more about the topology or the other routers.

At Step 2, router R1 receives the IP packet, and R1’s IP process makes a decision. R1 looks at the destination address (2.2.2.2), compares that address to its known IP routes, and chooses to forward the packet to router R2. This process of forwarding the IP packet is called IP routing (or simply routing). At Step 3, router R2 repeats the same kind of logic used by router R1. R2’s IP process will compare the packet’s destination IP address (2.2.2.2) to R2’s known IP routes and make a choice to forward the packet to the right, on to Bob.

All the CCNA exams cover IP fairly deeply. Practically half the chapters in this book discuss some feature that relates to addressing, IP routing, and how routers perform routing. TCP/IP Link Layer (Data Link Plus Physical)

The TCP/IP model’s original link layer defines the protocols and hardware required to deliver data across some physical network. The term link refers to the physical connections, or links, between two devices and the protocols used to control those links.

Just like every layer in any networking model, the TCP/IP link layer provides services to the layer above it in the model. When a host’s or router’s IP process chooses to send an IP packet to another router or host, that host or router then uses link-layer details to send that packet to the next host/router. Because each layer provides a service to the layer above it, take a moment to think about the IP logic related to Figure 1-10. In that example, host Larry’s IP logic chooses to send the IP packet to a nearby router (R1), with no mention of the underlying Ethernet. The Ethernet network, which implements link-layer protocols, must then be used to deliver that packet from host Larry over to router R1. Figure 1-11 shows four steps of what occurs at the link layer to allow Larry to send the IP packet to R1.

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Figure 1-11. Larry Using Ethernet to Forward an IP Packet to Router R1 Note
Figure 1-11 depicts the Ethernet as a series of lines. Networking diagrams often use this convention when drawing Ethernet LANs, in cases where the actual LAN cabling and LAN devices are not important to some discussion, as is the case here. The LAN would have cables and devices, like LAN switches, which are not shown in this figure. Figure 1-11 shows four steps. The first two occur on Larry, and the last two occur on router R1, as follows: Step 1. Larry encapsulates the IP packet between an Ethernet header and Ethernet trailer, creating an Ethernet frame.

Step 2. Larry physically transmits the bits of this Ethernet frame, using electricity flowing over the Ethernet cabling.
Step 3. Router R1 physically receives the electrical signal over a cable, and re-creates the same bits by interpreting the meaning of the electrical signals.
Step 4. Router R1 deencapsulates the IP packet from the Ethernet frame by removing and discarding the Ethernet header and trailer.
By the end of this process, the link-layer processes on Larry and R1 have worked together to deliver the packet from Larry to router R1.
Note
Protocols define both headers and trailers for the same general reason, but headers exist at the beginning of the message and trailers exist at the end.
The link layer includes a large number of protocols and standards. For example, the link layer includes all the variations of Ethernet protocols, along with several other LAN standards that were more popular in decades past. The link layer includes wide-area network (WAN) standards for different physical media, which differ significantly compared to LAN standards because of the longer distances involved in transmitting the data. This layer also includes the popular WAN standards that add headers and trailers as shown generally in Figure 111—protocols such as the Point-to-Point Protocol (PPP) and Frame Relay. Chapter 2, “Fundamentals of Ethernet LANs,†and Chapter 3, “Fundamentals of WANs,†further develop these topics for LANs and WANs, respectively.

In short, the TCP/IP link layer includes two distinct functions: functions related to the physical transmission of the data, plus the protocols and rules that control the use of the physical media. The five-layer TCP/IP model simply splits out the link layer into two layers (data link and physical) to match this logic.

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TCP/IP Model and Terminology
Before completing this introduction to the TCP/IP model, this section examines a few remaining details of the model and some related terminology.
Comparing the Original and Modern TCP/IP Models

The original TCP/IP model defined a single layer—the link layer—below the internetwork layer. The functions defined in the original link layer can be broken into two major categories: functions related directly to the physical transmission of data and those only indirectly related to the physical transmission of data. For example, in the four steps shown in Figure 1-11, Steps 2 and 3 were specific to sending the data, but Steps 1 and 4—encapsulation and deencapsulation—were only indirectly related. This division will become clearer as you read about additional details of each protocol and standard.

Today, most documents use a more modern version of the TCP/IP model, as shown in Figure 1-12. Comparing the two, the upper layers are identical, except a name change from “internet†to “network.†The lower layers differ in that the single link layer in the original model is split into two layers to match the division of physical transmission details from the other functions. Figure 1-12 shows the two versions of the TCP/IP model again, with emphasis on these distinctions.

Figure 1-12. Link Versus Data Link and Physical Layers
Data Encapsulation Terminology

As you can see from the explanations of how HTTP, TCP, IP, and Ethernet do their jobs, each layer adds its own header (and for data-link protocols, also a trailer) to the data supplied by the higher layer. The term encapsulation refers to the process of putting headers (and sometimes trailers) around some data. Many of the examples in this chapter show the encapsulation process. For example, web server Larry encapsulated the contents of the home page inside an HTTP header in Figure 1-6. The TCP layer encapsulated the HTTP headers and data inside a TCP header in Figure 1-7. IP encapsulated the TCP headers and the data inside an IP header in Figure 1-10. Finally, the Ethernet link layer encapsulated the IP packets inside both a header and a trailer in Figure 1-11.

The process by which a TCP/IP host sends data can be viewed as a five-step process. The first four steps relate to the encapsulation performed by the four TCP/IP layers, and the last step is the actual physical transmission of the data by the host. In fact, if you use the five-layer TCP/IP model, one step corresponds to the role of each layer. The steps are summarized in the following list:

Step 1. Create and encapsulate the application data with any required application layer headers. For example, the HTTP OK message can be returned in an HTTP header, followed by part of the contents of a web page.

Step 2. Encapsulate the data supplied by the application layer inside a transport layer header. For enduser applications, a TCP or UDP header is typically used. Step 3. Encapsulate the data supplied by the transport layer inside a network layer (IP) header. IP

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defines the IP addresses that uniquely identify each computer. Step 4. Encapsulate the data supplied by the network layer inside a data link layer header and trailer. This layer uses both a header and a trailer.

Step 5. Transmit the bits. The physical layer encodes a signal onto the medium to transmit the frame. The numbers in Figure 1-13 correspond to the five steps in this list, graphically showing the same concepts. Note that because the application layer often does not need to add a header, the figure does not show a specific application layer header.

Figure 1-13. Five Steps of Data Encapsulation: TCP/IP
Names of TCP/IP Messages

Finally, take particular care to remember the terms segment, packet, and frame and the meaning of each. Each term refers to the headers (and possibly trailers) defined by a particular layer and the data encapsulated following that header. Each term, however, refers to a different layer: segment for the transport layer, packet for the network layer, and frame for the link layer. Figure 1-14 shows each layer along with the associated term.

Figure 1-14. Perspectives on Encapsulation and “Dataâ€* * The letters LH and LT stand for link header and link trailer, respectively, and refer to the data link layer header and trailer.
Figure 1-14 also shows the encapsulated data as simply “data.†When focusing on the work done by a particular layer, the encapsulated data typically is unimportant. For example, an IP packet can indeed have a TCP header after the IP header, an HTTP header after the TCP header, and data for a web page after the HTTP header. However, when discussing IP, you probably just care about the IP header, so everything after the IP header is just called “data.†So, when drawing IP packets, everything after the IP header is typically shown simply as

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“data.â€
OSI Networking Model
At one point in the history of the OSI model, many people thought that OSI would win the battle of the networking models discussed earlier. If that had occurred, instead of running TCP/IP on every computer in the world, those computers would be running with OSI.

However, OSI did not win that battle. In fact, OSI no longer exists as a networking model that could be used instead of TCP/IP, although some of the original protocols referenced by the OSI model still exist. So, why is OSI even in this book? Terminology. During those years in which many people thought the OSI model would become commonplace in the world of networking (mostly in the late 1980s and early 1990s), many vendors and protocol documents started using terminology from the OSI model. That terminology remains today. So, while you will never need to work with a computer that uses OSI, to understand modern networking terminology, you need to understand something about OSI.

Comparing OSI and TCP/IP
The OSI model has many similarities to the TCP/IP model from a basic conceptual perspective. It has (seven) layers, and each layer defines a set of typical networking functions. As with TCP/IP, the OSI layers each refer to multiple protocols and standards that implement the functions specified by each layer. In other cases, just as for TCP/IP, the OSI committees did not create new protocols or standards, but instead referenced other protocols that were already defined. For example, the IEEE defines Ethernet standards, so the OSI committees did not waste time specifying a new type of Ethernet; it simply referred to the IEEE Ethernet standards. Today, the OSI model can be used as a standard of comparison to other networking models. Figure 1-15 compares the seven-layer OSI model with both the four-layer and five-layer TCP/IP models.

Figure 1-15. OSI Model Compared to the Two TCP/IP Models
Next, this section will examine two ways in which we still use OSI terminology today: to describe other protocols and to describe the encapsulation process. Along the way, the text will briefly examine each layer of the OSI model.

Describing Protocols by Referencing the OSI Layers
Even today, networking documents often describe TCP/IP protocols and standards by referencing OSI layers, both by layer number and layer name. For example, a common description of a LAN switch is “layer 2 switch,†with “layer 2†referring to OSI layer 2. Because OSI did have a well-defined set of functions associated with each of its seven layers, if you know those functions, you can understand what people mean when they refer to a product or function by its OSI layer.

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For another example, TCP/IP’s original internet layer, as implemented mainly by IP, equates most directly to the OSI network layer. So, most people say that IP is a network layer protocol, or a Layer 3 protocol, using OSI terminology and numbers for the layer. Of course, if you numbered the TCP/IP model, starting at the bottom, IP would be either Layer 2 or 3, depending on what version of the TCP/IP model you care to use. However, even though IP is a TCP/IP protocol, everyone uses the OSI model layer names and numbers when describing IP or any other protocol for that matter.

The claim that a particular TCP/IP layer is similar to a particular OSI layer is a general comparison, but not a detailed comparison. The comparison is a little like comparing a car to a truck: Both can get you from point A to point B, but they have many specific differences, like the truck having a truck bed in which to carry cargo. Similarly, both the OSI and TCP/IP network layers define logical addressing and routing. However, the addresses have a different size, and the routing logic even works differently. So the comparison of OSI layers to other protocol models is a general comparison of major goals, and not a comparison of the specific methods. OSI Layers and Their Functions

Cisco requires that CCENTs demonstrate a basic understanding of the functions defined by each OSI layer, as well as remember the names of the layers. It is also important that, for each device or protocol referenced throughout the book, you understand which layers of the OSI model most closely match the functions defined by that device or protocol.

Today, because most people happen to be much more familiar with TCP/IP functions than with OSI functions, one of the best ways to learn about the function of different OSI layers is to think about the functions in the TCP/IP model and to correlate those with the OSI model. If you use the five-layer TCP/IP model, the bottom four layers of OSI and TCP/IP map closely together. The only difference in these bottom four layers is the name of OSI Layer 3 (network) compared to the original TCP/IP model (internet). The upper three layers of the OSI reference model (application, presentation, and session—Layers 7, 6, and 5) define functions that all map to the TCP/IP application layer. Table 1-4 defines the functions of the seven layers. Table 1-4. OSI Reference Model Layer Definitions

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Table 1-5 lists a sampling of the devices and protocols covered in the CCNA exams and their comparable OSI layers. Note that many network devices must actually understand the protocols at multiple OSI layers, so the layer listed in Table 1-5 actually refers to the highest layer that the device normally thinks about when performing its core work. For example, routers need to think about Layer 3 concepts, but they must also support features at both Layers 1 and 2.

Table 1-5. OSI Reference Model—Example Devices and Protocols

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Besides remembering the basics of the features of each OSI layer (as in Table 1-4), and some example protocols and devices at each layer (as in Table 1-5), you should also memorize the names of the layers. You can simply memorize them, but some people like to use a mnemonic phrase to make memorization easier. In the following three phrases, the first letter of each word is the same as the first letter of an OSI layer name, in the order specified in parentheses:

All People Seem To Need Data Processing (Layers 7 to 1)
Please Do Not Take Sausage Pizzas Away (Layers 1 to 7)
Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7) OSI Layering Concepts and Benefits
While networking models use layers to help humans categorize and understand the many functions in a network, networking models use layers for many reasons. For example, consider another postal service analogy. A person writing a letter does not have to think about how the postal service will deliver a letter across the country. The postal worker in the middle of the country does not have to worry about the contents of the letter. Likewise, networking models that divide functions into different layers enable one software package or hardware device to implement functions from one layer, and assume that other software/hardware will perform the functions defined by the other layers.

The following list summarizes the benefits of layered protocol specifications:

Less complex: Compared to not using a layered model, network models break the concepts into smaller parts.
Standard interfaces: The standard interface definitions between each layer allow multiple vendors to create products that fill a particular role, with all the benefits of open competition. Easier to learn: Humans can more easily discuss and learn about the many details of a protocol specification.

Easier to develop: Reduced complexity allows easier program changes and faster product development. Multivendor interoperability: Creating products to meet the same networking standards means that computers and networking gear from multiple vendors can work in the same network. Modular engineering: One vendor can write software that implements higher layers—for example, a web browser—and another vendor can write software that implements the lower layers—for example, Microsoft’s built-in TCP/IP software in its OSs.

OSI Encapsulation Terminology

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Like TCP/IP, each OSI layer asks for services from the next lower layer. To provide the services, each layer makes use of a header and possibly a trailer. The lower layer encapsulates the higher layer’s data behind a header. The final topic of this chapter explains some of the terminology and concepts related to OSI encapsulation.

The TCP/IP model uses terms such as segment, packet, and frame to refer to various layers and their respective encapsulated data (refer to Figure 1-13). OSI uses a more generic term: protocol data unit (PDU). A PDU represents the bits that include the headers and trailers for that layer, as well as the encapsulated data. For example, an IP packet, as shown in Figure 1-14, using OSI terminology, is a PDU. In fact, an IP packet is a Layer 3 PDU (abbreviated L3PDU) because IP is a Layer 3 protocol. So, rather than use the terms segment, packet, or frame, OSI simply refers to the “Layer x PDU†(LxPDU), with “x†referring to the number of the layer being discussed.

Figure 1-16 represents the typical encapsulation process, with the top of the figure showing the application data and application layer header and the bottom of the figure showing the L2PDU that is transmitted onto the physical link.

Figure 1-16. OSI Encapsulation and Protocol Data Units

Exam Preparation Tasks
Review All the Key Topics
Review the most important topics from this chapter, noted with the Key Topic icon. Table 1-6 lists these key topics and where each is discussed.

Table 1-6. Key Topics for Chapter 1

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Definitions of Key Terms
After your first reading of the chapter, try to define these key terms, but do not be concerned about getting them all correct at that time. Chapter 30 directs you in how to use these terms for late-stage preparation for the exam. adjacent-layer interaction

deencapsulation
encapsulation
frame
networking model
packet
protocol data unit (PDU)
same-layer interaction
segment

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Cite This Document

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