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Introduction to Computer Networking Concepts

computer technologyThis chapter gives you a light-hearted perspective about networks, how they were originally created, and why networks work the way they do. Although no specific fact from this chapter happens to be on either of the CCNA exams, this chapter helps you prepare for the depth of topics you will start to read about in Chapter 2, “The TCP/IP and OSI Networking Models.” If you are brand new to networking, this short introductory chapter will help you get ready for the details to follow. If you already understand the basics of TCP/IP, Ethernet, switches, routers, IP addressing, and the like, go ahead and skip on to Chapter 2. The rest of you will probably want
to read through this short introductory chapter before diving into the details.
Perspectives on Networking.

So, you are new to networking. You might have seen or heard about different topics relating to networking, but you are only just now getting serious about learning the details. 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, that perspective is as a dialup user of the Internet. Others might use a computer at a job or at school; that computer is typically connected to a network via some cable. Figure 1-1 shows the basic end-user perspective of networking.

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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.
From the end-user perspective, whether at home, at work, or at school, what happens behind the wall plug is magic. Just as most people do not really understand how cars work, how TVs work, and so on, most people who use networks do not understand how they work. Nor do they want to! But if you have read this much into Chapter 1, you obviously have a little more interest in networking than an end user. By the end of this book, you will have a pretty thorough understanding of what’s behind that wall plug.
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.

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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.
The Flintstones Network: The First Computer Network?
The Flintstones are a cartoon family that, according to the cartoon, lived in prehistoric times.
Because I want to discuss the thought process behind some imaginary initial networking standards, the Flintstones seem to be the right group of people to put in the example. 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.

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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.

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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 (10 4) 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.

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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 fourdigit 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.

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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 knowLike 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 design 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.
Now on to the real chapters, with real protocols and devices, with topics that you could seeon the CCNA INTRO exam.


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