You have seen several examples of encapsulation in this chapter already. The web server encapsulated the home page inside an HTTP header in Figure 2-2. The TCP layer encapsulated the HTTP headers and data inside a TCP header in Figure 2-3. IP encapsulated the TCP headers and the data inside an IP header in Figure 2-4. Finally, the network interface layer encapsulated the IP packets inside both a header and a trailer in Figure 2-5.

You can think about the complete process of data encapsulation with TCP/IP as a five-step process. In fact, previous CCNA exams referred to a specific five-step process for encapsulation. This included the typical encapsulation by the application, transport, network, and network interface (referred to as data link) layers as Steps 1 through 4 in the five-step process. The fifth step was the physical layer’s transmission of the bit stream. In case any questions remain in the CCNA question database referring to a five-step encapsulation process, the following list provides the details and explanation. Regardless, the ideas behind the process apply to any networking model and how it encapsulates data:

Step 1 Create the application data and headers—This simply means that the application has data to send.
Step 2 Package the data for transport—In other words, the transport layer (TCP or UDP) creates the transport header and places the data behind it.
Step 3 Add the destination and source network layer addresses to the data. — The network layer creates the network header, which includes the network layer addresses, and places the data behind it.
Step 4 Add the destination and source data link layer addresses to the data — The data link layer creates the data link header, places the data behind it, and places the data link trailer at the end.
Step 5 Transmit the bits—The physical layer encodes a signal onto the medium to transmit the frame.

This five-step process happens to match the TCP/IP network model very well. Figure 2-6 depicts the concept; the numbers shown represent each of the five steps.

Figure 2-6 Five Steps of Data Encapsulation—TCP/IP

* The letters LH and LT stand for link header and link trailer, respectively, and refer to the data link layer header and trailer.

When each layer encapsulates data given to it from the next higher layer, that layer does not really care about the details of the data. Figure 2-7 shows the encapsulated data from the perspective of the transport, internetwork, and data link (network interface) layers.

Figure 2-7 Perspectives on Encapsulation and “Data”

Each layer treats the data given to it by the next higher layer simply as “data.” For instance, IP just wants to transport what TCP gives it—IP does not really care what is inside the data. So, the IP packet shown in the figure shows the rest of the bits as data, meaning that IP does not care that the data field looks like the TCP segment above it in the figure.

Also notice the specific terms used for the framing as it exists at each layer, as shown in the figure. Throughout this book and on the CCNA exams, the term frame defines all the encapsulated data. The term packet includes the IP header but not any data link headers. Finally, the term segment includes the TCP or UDP header but not the IP header or data link header or trailer.

Chapter 3, “Data Link Layer Fundamentals: Ethernet LANs,” and Chapter 4, “Fundamentals of WANs,” cover more details about the TCP/IP network interface layer.

Just like every layer in any networking model, the TCP/IP network interface layer provides services to the layer above it in the model. The best way to understand the basics of the TCP/IP network interface layer is to examine the services that it provides to IP.

IP relies on the network interface layer to deliver IP packets across each physical network. IP understands the overall network topology, things such as which routers are connected to each other, which host computers are connected to which networks, and what the IP addressing scheme looks like. However, the IP protocol purposefully does not include the details about each of the underlying physical networks. Therefore, the Internet layer, as implemented by IP, uses the services of the network interface layer to deliver the packets over each physical network, respectively.

The network interface layer includes a large number of protocols. For instance, the network interface layer includes all the variations of Ethernet protocols and other LAN standards. This layer also includes the popular WAN standards, such as the Point-to-Point Protocol (PPP) and Frame Relay. The same familiar network is shown in Figure 2-5, with Ethernet and PPP used as the two network interface layer protocols.

TCP/IP Network Interface Layer

Figure 2-5 Ethernet and PPP Services Provided to IP

To fully appreciate Figure 2-5, first think a little more deeply about how IP accomplishes its goal of delivering the packet from Bob to Larry. Bob wants to send the IP packet to Larry, but it must first do so by sending the packet to R2. Bob uses Ethernet to get the packet to R2. At R2, R2 strips the Ethernet header and trailer from the IP packet. To get the IP packet from R2 to R1, R2 does not need to use Ethernet—it instead needs to use the PPP serial link. To send the IP packet from R2 to R1, R2 needs to place a PPP header in front of the IP packet and a PPP trailer at the end. Similarly, after the packet is received by R1, R1 removes the PPP header and trailer because PPP’s job is to get the IP packet across the serial link. R1 then decides that it should forward the packet over the Ethernet to Larry. To do so, R1 adds a brand-new Ethernet header and trailer to the packet and forwards it to Larry.

In effect, IP uses the network interface layer protocols to deliver the IP packet to the next router or host, with each router repeating the process until the packet arrives at the destination. Each network interface protocol uses headers to encode the information needed to successfully deliver the data across the physical network, much like other layers use headers to achieve their goals.

CAUTION
Many people describe the network interface layer of the TCP/IP model as two layers, the data link layer and the physical layer. The reasons for the popularity of these alternate terms are explained in the section covering OSI because the terms originated with the OSI model.

In short, the TCP/IP Network Interface layer includes the protocols, cabling standards, headers and trailers that define how to send data across a wide variety of types of physical networks.

Imagine that you just wrote a letter to your favorite person on the other side of the country and that you also wrote a letter to someone on the other side of town. It’s time to send the letters. Is there much difference in how you treat each letter? Not really. You put different addresses on the envelope for each letter because the letters need to go to two different places. You put stamps on both letters and put them in the same mailbox. The postal service takes care of all the details of figuring out how to get each letter to the right place—whether it is across town or across the country.

Inside the postal service, both letters are processed. One letter gets sent to another post office, then another, and so on, until the letter gets delivered across the country. The local letter might go to the post office in your town and then simply be delivered to your friend across town, without going to another post office.

So what does this all matter to networking? Well, the internetwork layer of the TCP/IP networking model, the Internet Protocol (IP), works much like the postal service. IP defines addresses so that each host computer can have a different IP address, just like 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 (ingenious name, huh?) can choose where to send packets of data so that they are delivered to the correct destination. Just like the postal service created the necessary post offices, sorting machines, trucks, and personnel to deliver the mail, the internetwork layer defines much of the details needed to implement the necessary networking infrastructure.

Chapter 5, “Fundamentals of IP,” describes the TCP/IP Internetwork layer further, with other details scattered throughout the book. But to help you understand the basics of the internetwork layer, take a look at Bob’s request for Larry’s home page, now with some information about IP, in Figure 2-4.

First, some basic information about the figure will help. The LAN cabling details are not important for this example, so both LANs simply are represented by the lines shown near Bob and Larry, respectively. When Bob sends the data, he is sending an IP packet, which includes the IP header, the transport layer header (TCP, in this example), the application header (HTTP, in this case), and any application data (none, in this case). The IP header includes both a source and a destination IP address field, with Larry’s IP address as the destination address and Bob’s as the source.

Figure 2-4 IP Services Provided to TCP

Bob sends the packet to R2, which makes a routing decision. R2 chooses to send the packet to R1 because the destination address of the packet is 1.1.1.1, and R1 knows enough about the network topology to know that 1.1.1.1 (Larry) is on the other side of R1. Similarly, when R1 gets the packet, it forwards the packet over the Ethernet to Larry. And if the link between R2 and R1 fails, IP allows R2 to learn of the alternate route through R3 to reach 1.1.1.1.

IP defines logical addresses, called IP addresses, that allow each TCP/IP speaking device (called IP hosts) to communicate. It also defines routing—the process of how a router should forward, or route, packets of data. Other protocol specifications, like OSI, have different protocols that also define addressing and routing.

Both CCNA exams cover IP fairly deeply. For the INTRO exam, this book’s Chapter 5 covers more of the basics, and Chapters 12, “IP Addressing and Subnetting,” through 14, “Introduction to Dynamic Routing Protocols,” cover many of the details.

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. For example, in Figure 2-2, Bob and Larry used HTTP to transfer the home page from Larry to Bob. But what would have happened if Bob’s HTTP get request was lost in transit through the TCP/IP network? Or, what would have happened if Larry’s response, which includes the contents of the home page, was lost? Well, the page would not show up in Bob’s browser, as you might expect.

So, TCP/IP needs a mechanism to guarantee delivery of data across a network. TCP provides that feature by using acknowledgments. Figure 2-3 outlines the basic acknowledgment logic.

As Figure 2-3 shows, the HTTP software asks for TCP to reliably deliver the HTTP get request. TCP sends the HTTP data from Bob to Larry, and the data arrives successfully.

Larry’s TCP software acknowledges receipt of the data and also gives the HTTP get request to the web server software. The reverse happens with Larry’s response, which also arrives at Bob successfully.

Figure 2-3 TCP Services Provided to HTTP

Of course, the benefits of TCP error recovery cannot be seen unless the data is lost. “Fundamentals of TCP and UDP” covers TCP, including error recovery, in detail. For now, assume that if either transmission had been lost, that HTTP would not be concerned, and that TCP would resend the data and ensure that it was received successfully.

This example outlines the concepts of how adjacent layers in a networking model work together on the same computer. The higher-layer protocol (HTTP) needs to do something it cannot do (error recovery). So, the higher layer asks for the next lower-layer protocol (TCP) to perform the service, and the next lower layer performs the service. The lower layer provides a service to the layer above it.

Table 2-3 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 2-3 Summary: Same-Layer and Adjacent-Layer Interactions

The TCP/IP transport layer provides services to the various application layer protocols. Error recovery, as performed by TCP, is one feature. This layer also provides other functions.

All the examples describing the application and transport layers ignored many details relating to the physical network. The application and transport layers purposefully were defined to work the same, way whether the endpoint host computers were on the same LAN or were separated by the Internet. The lower two layers of TCP/IP, the internetwork layer and the network interface layer, must understand the underlying physical network because they define the protocols used to deliver the data from one host to another.

What really happens to allow that web page to appear on your web browser? These next few sections take a high-level look at what happens behind the scene.

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 that in Figure 2-1.

Figure 2-1 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 Larry’s 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 the web browser window.

Taking a closer look, this example uses two TCP/IP application layer protocols. First, the request for the file and the actual transfer of the file are performed according to the Hypertext Transfer Protocol (HTTP). Many of you have probably noticed that most web sites’ URLs (Universal Resource Locators, the text that identifies a web server and a particular web page) begin with the letters “http,” to imply that HTTP will be used to transfer the web pages.

The other protocol used is the Hypertext Markup Language (HTML). HTML defines how Bob’s web browser should interpret the text inside the file he just received. For instance, the file might contain directions about making certain text be a certain size, color, and so on. In most cases, it also includes directions about other files that Bob’s web browser should get things such as graphics images and animation. HTTP would then be used to get those additional files from Larry, the web server.

A closer look at how Bob and Larry cooperate in this example reveals some details about how networking protocols work. Consider Figure 2-2, which simply revises Figure 2-1, showing the locations of HTTP headers and data.

Figure 2-2 HTTP Get Request and HTTP Reply

To get the web page from Larry, Bob sends something called an HTTP header to Larry. This header includes the command 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.

The response from Larry includes an HTTP header as well, with something as simple as “OK” returned in the header. In reality, it includes an HTTP return code. For instance, if you have ever used the web, and a web page that you looked for was not found, then you received an HTTP 404 “not found” error, which means that you received an HTTP return code of 404. When the requested file is found, the return code is 0, meaning that the request is being processed.

This simple example between Bob and Larry introduces one of the most important general concepts behind networking models: When a particular layer wants to communicate with the same layer on another computer, the two computers use headers to hold the information that they want to communicate. The headers are part of what is transmitted between the two computers. This process is called same-layer interaction.

The application layer protocol (HTTP, in this case) on Bob is communicating with Larry’s application layer. They each do so by creating and sending application layer headers to each other—sometimes with application data following the header and sometimes not, as seen in Figure 2-2. Regardless of what the application layer protocol happens to be, they all use the same general concept of communicating with the same layer on the other computer using application layer headers.

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 rather it defines services that applications need—like the ability to transfer a file in the case of HTTP. In short, the application layer provides an interface between software running on a computer and the network itself.

TCP/IP defines a large collection of protocols that allow computers to communicate. TCP/IP defines the details of each of these protocols inside document called Requests For Comments (RFCs). By implementing the required protocols defined in TCP/IP RFCs, a computer can be relatively confident that it can communicate with other computers that also implement TCP/IP.

An easy comparison can be made between telephones and computers that use TCP/IP. I can go to the store and buy a phone from one of a dozen different vendors. When I get home, I plug the phone in to the wall socket, and it works. The phone vendors know the standards for phones in their country and build their phones to match those standards. Similarly, a computer that implements the standard networking protocols defined by TCP/IP can communicate with other computers that also use the TCP/IP standards.

Like other networking architectures, TCP/IP classifies the various protocols into different categories. Table 2-2 outlines the main categories in the TCP/IP architectural model.

Table 2-2 TCP/IP Architectural Model and Example Protocols

The TCP/IP model represented in column 1 of the table lists the four layers of TCP/IP, and column 2 of the table lists several of the most popular TCP/IP protocols. If someone makes up a new application, the protocols used directly by the application would be considered to be application layer protocols. When the World Wide Web (WWW) was first created, a new application layer protocol was created for the purpose of asking for web pages and receiving the contents of the web pages. Similarly, the network interface layer includes protocols and standards such as Ethernet. If someone makes up a new type of LAN, those protocols would be considered to be a part of the networking interface layer. In the next several sections, you will learn the basics about each of these four layers in the TCP/IP architecture and how they work together.

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 rather it defines services that applications need - like the ability to transfer a file in the case of HTTP. In short, the application layer provides an interface between software running on a computer and the network itself.

TCP/IP Protocol Architecture:

1. TCP/IP Application Layer
2. TCP/IP Transport Layer
3. TCP/IP Internetwork Layer
4. TCP/IP Network Interface Layer

Fred is the president of FredCo, where his wife (Wilma), buddy (Barney), and buddy’s wife (Betty) all work. They all have phones and computers, but they have no network because no one has ever made up the idea of a network before. Fred sees all his employees running around giving each other disks with files on them, and it seems inefficient. So, Fred, being a visionary, imagines a world in which people can connect their computers somehow and exchange files, without having to leave their desks. The (imaginary) first network is about to be born.

Fred’s daughter, Pebbles, has just graduated from Rockville University and wants to join the family business. Fred gives her a job, with the title First-Ever Network Engineer. Fred says to Pebbles, “Pebbles, I want everyone to be able to exchange files without having to get up from their desks. I want them to be able to simply type in the name of a file and the name of the person, and poof! The file appears on the other person’s computer. And because everyone changes departments so often around here, I want the workers to be able to take their PCs with them and just have to plug the computer into a wall socket so that they can send and receive files from the new office they moved to. I want this network thing to be like the electrical power thing your boyfriend, Bam Bam, created for us last year—a plug in the wall near every desk, and if you plug in, you’re on the network!”

Pebbles first decides to do some research and development. If she can get two PCs to transfer files in a lab, then she ought to be able to get all the PCs to transfer files, right? She writes a program called Fred’s Transfer Program, or FTP, in honor of her father.

The program uses a new networking card that Pebbles built in the lab. This networking card uses a cable with two wires in it—one wire to send bits and one to receive bits. Pebbles puts one card in each of the two computers and cables the computers together with a cable with two wires in it. The FTP software on each computer sent the bits that comprised the files using the networking cards. If Pebbles types a command like ftp send filename, the software transfers the file called filename to the computer at the other end of the cable. Figure 1-3 depicts the first network test at FredCo.

Figure 1-3 Two PCs Transfer Files in the Lab

Note that because each networking card uses wire 1 to send bits and wire 2 to receive bits, the cable used by Pebbles connects wire 1 on PC1 to wire 2 on PC2, and vice versa. That way, both cards can send using wire 1, and it will enter the other PC on the other PC’s wire 2.

Bam Bam happens by to give Pebbles some help after hearing about the successful test. “I’m ready to start deploying the network!” she claims. Bam Bam, the wizened one-year veteran of FredCo who graduated from Rockville U. a year before Pebbles, starts asking some questions. “What happens when you want to connect three computers together?” he asks. Pebbles explains that she can put two networking cards in each computer and cable each computer to each other. “So what happens when you connect 100 computers to the network—in each building?” Hmmm…. Pebbles then realizes that she has a little more work to do. She needs a scheme that allows her network to scale to more than two users. Bam Bam goes on, “We ran all the electrical power cables from the wall plug at each cube back to the broom closet. We just send electricity from the closet out to the wall plug near every desk. Maybe if you did something similar, you can find a way to somehow make it all work.”

With that bit of input, Pebbles has all the inspiration she needs. Emboldened by the fact that she had already created the world’s first PC networking card, she decides to create a device that will allow cabling similar to Bam Bam’s electrical cabling plan. Pebble’s solution to this first major hurdle is shown in Figure 1-4.

Figure 1-4 Star Cabling to a Repeater

Pebbles follows Bam Bam’s advice about the cabling. However, she needs a device into which she can plug the cables—something that will take the bits sent by a PC, and reflect, or repeat, the bits back to all the other devices connected to this new device. Because the networking cards send bits using wire 1, Pebbles builds this new device so that when it receives bits coming in wire 1 on one of its ports, it will repeat the same bits—but out wire 2 on all the other ports, so the other PCs get those bits on the receive wire. (Therefore, the cabling does not have to swap wires 1 and 2—this new device takes care of that.) And because she is making this up for the very first time in history, she needs to decide on a name for this new device: She names the device a hub.

Before deploying the first hub and running a bunch of cables, Pebbles does the right thing:
She tests it in a lab, with three PCs connected to the world’s first hub. She starts FTP on PC1, transfers the file called recipe.doc, and sees a window pop up on PC2 saying that the file was received, just like normal. “Fantastic!” she thinks—until she realizes that PC3 also has the same pop-up window on it. She has transferred the file to both PC2 and PC3! “Of course!” she thinks. “If the hub repeats everything out every cable connected to it, then when FTP sends a file, everyone will get it. I need a way for FTP to send a file to a specific PC!”

At this point, Pebbles thinks of a few different options. First, she thinks that she will give each computer the same name as the first name of the person using the computer. She will then change FTP to put the name of the PC that the file was being sent to in front of the file contents. In other words, to send her mom a recipe, she will use the ftp Wilma recipe.doc command. So, each PC will receive the bits because the hub repeats the signal to everyone connected to it, but only the PC whose name is the one in front of the file should actually create the file. Then her Dad walks in: “Pebbles, I want you to meet Barney Fife, our new head of security. He’ll need a network connection as well—you are going to be finished soon, right?”

So much for using first names for the computers: There are now two people named Barney at FredCo. Pebbles, being mathematically inclined and in charge of creating all the hardware, decides on a different approach. “I’ll put a unique address on each networking card—a 4-digit decimal number,” she exclaims. Because Pebbles created all the cards, she will make sure that the number used on each card is unique. Also, with a 4-digit number, she will never run out of unique numbers—she has 10,000 (104) to choose from and only 200 employees at FredCo.

By the way, because she’s making all this up for the very first time, she calls these built-in numbers on the cards addresses. When anyone wants to send a file, they can just use the ftp command, but with a number instead of a name. For instance, ftp 0002 recipe.doc will send the recipe.doc file to the PC whose network card has the address 0002. Figure 1-5 depicts the new environment in the lab.

Figure 1-5 The First Network Addressing Convention

Now, with some minor updates to the Fred Transfer Program, the user can type ftp 0002 recipe.doc to send the file recipe.doc to the PC with address 0002. Pebbles tests the software and hardware in the lab again, and only PC2 receives the file when it is sent to PC2. When she sends the file to 0003, only PC3 receives the file. She’s now ready to deploy the first computer network.

Pebbles now needs to build all the hardware needed. She first creates 200 network cards, each with a unique address. She installs the FTP program on all 200 PCs and installs the cards in each PC. Then she goes back to the lab and starts planning how many cables she will need and how long each cable should be. Then she realizes that she will need to run some cables a long way. Even if she puts the hub in the bottom floor of building A, the PCs on the fifth floor of building B will need a really long cable to connect to the hub. Cables cost money, and the longer the cable is, the more expensive the cable is. Besides, she has not yet tested the network with longer cables; she has been using cables that are only a couple of meters long.

Bam Bam happens by and sees that Pebbles is stressed. Pebbles vents a little: “Daddy wants this project finished, and you know how demanding he is. And I didn’t think about how long the cables will be—I’ll be way over budget. And I’ll be running cables for weeks!” Bam Bam, being a little less stressed, having just come from a workout during lunch break at the club, knows that Pebbles already has the solution—she was too stressed to see it. Of course, the solution is not terribly different from how Bam Bam solved a similar problem with the electrical cabling last year. “Those hubs repeat everything they hear, right? So, why not make a bunch of hubs. Put one hub on each floor, and run cables from all the PCs. Then run a cable from the hub on each floor to a hub on the first floor Then, run one cable between the two main hubs in the two buildings. Because they repeat everything, every PC should receive the signal when just one PC sends, whether they are attached to the same hub or are four hubs away.” Figure 1-6 depicts Bam Bam’s suggested design.

Figure 1-6 Per-Floor Hubs, Connected Together

Pebbles loves the idea. She builds and connects the new hubs in the lab, just to prove the concept. It works! She makes the (now shorter) cables, installs the hubs and cables, and is ready to test. She goes to a few representative PCs and tests, and it all works! The first network has now been deployed.

Wanting to surprise Poppa Fred, Pebbles writes a memo to everyone in the company, telling them how to use the soon-to-be-famous Fred Transfer Program to transfer files. Along with the memo, she puts a list of names of people and the four-digit network address to be used to send files to each PC. She puts the memos in everyone’s mail slot and waits for the excitement to start.

Amazingly, it all works. The users are happy. Fred treats Pebbles and Bam Bam to a nice dinner—at home, cooked by Wilma, but a good meal nonetheless.

Pebbles thinks she did it—created the world’s first computer network, with no problems until a few weeks pass. “I can’t send files to Fred anymore!” exclaims Barney Rubble. “Ever since Fred got that new computer, he’s too busy to go bowling, and now I can’t even send him files telling him how much we need him back on the bowling team!” Then it hits Pebbles—Fred had just gotten a new PC and a new networking card. Fred’s network address had changed. Or what happens if the card fails and it has to be replaced? The address changes.

About that time, Wilma comes in to say hi. “I love that new network thing you built. Betty and I can type each other notes, put them in a file, and send them anytime. It’s almost like working on the same floor!” she says. “But I really don’t remember the numbers so well. Couldn’t you make that FTP thing work with names instead of addresses?”

In a fit of inspiration, Pebbles sees the answer to the first problem in the solution to her mom’s problem. “I’ll change FTP to use names instead of addresses. I’ll make everyone tell me what name they want to use—maybe Barney Rubble will use BarneyR, and Barney Fife will use BarneyF, for instance. I’ll change FTP to accept names as well as numbers. Then I’ll tell FTP to look in a table that I will put on each PC that correlates the names to the numeric addresses. That way, if I ever need to replace a LAN card, all I have to do is update the list of names and addresses and put a copy on everyone’s PC, and no one will know that anything has changed!” Table 1-2 lists Pebbles first name table.

Pebbles tries out the new FTP program and name/address table in the lab, and it works. She deploys the new FTP software, puts the name table on everyone’s PC, sends another memo and now she can accommodate changes easily by separating the physical details, such as addresses on the networking cards, from what the end users need to know.

Like all good network engineers, Pebbles thought through the design and tested in a lab before deploying the network. For the problems she did not anticipate, she found a reasonable solution to get around the problem.

So ends the obviously contrived imaginary first computer network. What purpose did this silly example really serve? First, you have now been forced to think about some basic issues that confronted the people who created the networking tools that you will be learning about for the CCNA exams. Although the example with Pebbles might have been fun, the problems that she faced are the same problems faced—and solved—by the people who created the original networking protocols and products.

The other big benefit to this story, particularly for those of you brand new to networking, is that you already know some of the more important concepts in networking:

• Ethernet networks use cards inside each computer.
• The cards have unique addresses, similar to Pebble’s networking cards.
• Ethernet cables connect PCs to Ethernet hubs—hubs that repeat each received signal out all other ports.
• The cabling is typically run in a star configuration—in other words, all cables run from a cubicle to a wiring (not broom!) closet.
• Applications such as the contrived Fred Transfer Program or the real-life File Transfer Protocol (FTP) ask the underlying hardware to transfer the contents of files. Users can use names—for instance, you might surf a web site called www.myfavoritewebsite.org—but the name gets translated into the correct address.

Figure 1-1 End-User Perspective on Networks

The top part of the figure shows a typical dialup user of the Internet. The user has a PC, and the user plugs in the phone line from the wall into a modem in a PC. By dialing the right phone number, the user connects to the Internet. After connecting, the user can send e-mail, browse web sites, and use other tools and applications as well.

Similarly, an employee of a company or a student at a university views the world as a connection through a wall plug. Typically, this connection uses a type of local-area network (LAN) called Ethernet. Instead of a phone cord between a PC modem and the wall plug at your house, you have an Ethernet cable between a PC Ethernet card and a wall plug near where you are sitting at work or at school. The Ethernet connection does not require the PC to “dial” a phone number—it’s always there waiting to be used, similar to the power outlet.

The concepts, protocols, and devices covered on the CCNA exam are used to help build the network cloud shown in Figure 1-1. However, the CCNA exam focuses on technology that is used to build a network at a single company or school. These same technologies are used to build the Internet, but the CCNA exam topics focus on things that matter most to what Cisco calls “enterprise” networks—networks owned by a single enterprise or company. Figure 1-2 shows an alternative view of the world of networking, with several enterprise networks.

Figure 1-2 Enterprise Networks and the Internet

When you go to your school or your job and connect to “the network,” you are most likely connecting to the private network, or enterprise network, for that school or company. That network, in turn, is connected to the Internet. Conversely, if you dial into some Internet service provider (ISP) from home, you are not connected to an Enterprise network, but you are connected directly to the Internet. However, if you then use a web browser to browse some web site, the web site itself might be inside that company’s enterprise network.

In either case, practically every company or school that uses computers also has an enterprise network. To communicate, many enterprise networks connect to the Internet. The Internet itself is really a collection of ISPs that, in turn, connect to each other. By having the various enterprise networks connect to the Internet, most computer users around the world can use applications to communicate with each other—worldwide.

The CCNA exams focus on the technology used to build enterprise networks, with some coverage of technology more often used in the Internet. However, a lot of the protocols and concepts used in an enterprise network also happen inside the Internet. Because CCNA topics encompass the typical features found in enterprise networks, and because a much larger number of people work on enterprise networks than ISP networks, most of the examples in this book focus on enterprise networks.

Most of the details about standards for enterprise networks were created in the last quarter of the 20th century. You might have gotten interested in networking after most of the conventions and rules used for basic networking were created—if so, you missed out on the opportunity to help create the standards. However, taking the time to pause and think about what you would do if you were creating these standards can be helpful. The next section takes you through a somewhat silly example, but with real value in terms of thinking through some of the basic concepts behind enterprise networking and some of the design trade-offs.

1. Crossover wiring diagram
A crossover cable can be used to:
a) Connect 2 computers directly.
b) Connect a router’s LAN port to a switch/hub’s normal port. (normally used for expanding network)
c) Connect 2 switches/hubs by using normal port in both switches/hubs.

2. Straight Through wiring diagram
A straight through cabling can be used to:
a) Connect a computer to a switch/hub’s normal port.
b) Connect a computer to a cable/DSL modem’s LAN port.
c) Connect a router’s WAN port to a cable/DSL modem’s LAN port.
d) Connect a router’s LAN port to a switch/hub’s uplink port. (normally used for expanding network)
e) Connect 2 switches/hubs with one of the switch/hub using an uplink port and the other one using normal port.

3. Rollover wiring diagram
Rollover cabling is most commonly used to connect a computer terminal to a router’s console port. This cable is typically flat (and has a light blue color) to help distinguish it from other types of network cabling. It gets the name rollover because the pinouts on one end are literally rolled over when RJ45 plugs are used at both ends.