At the beginning, it's easy. You (or your employer or your spouse) bring a computer into your office or your home, and everything is right there: word processing files, financial records, email, music and video, maybe some games, and a connection to the Internet. It's all in one place. Love it or hate it, that computer has become an important part of the way you work and play. In fact, it's so important and so convenient that you eventually decide to add another computer; it might be a laptop that you can carry from one place to another, or maybe a second desktop machine that allows more than one person to use a computer at the same time. And that's when the trouble starts.
Shortly after you get that additional computer, you will discover that something—a text file or a picture you need for a report, or a music file you want to play, or the modem that connects you to the Internet—is located on the other computer. You have to copy files to a portable disk or a flash drive when you have something to print and carry it to the computer connected to your printer; when you want to scan something you must go to the computer with the scanner; and when you want to connect to the Internet, you must either use the computer with the high-speed connection or wait until another family member has finished using the telephone so you can dial in. Using the computer has risen to a whole new level of inconvenience.
Any time you (or your family or business) use more than one computer, something you want—a file, a printer, or some other resource—is likely to be located on or connected to the computer you're not currently using; it's inevitable that something you need on this computer is stored on or connected to that computer. The solution to this problem is easy: Simply connect the computers and allow them to share.
Congratulations. You have just created a computer network.
Two or more computers connected through wires, radio signals, flashing lights, or any combination of those and other methods form a network that you can use to send and receive instructions and files from one computer to another. Whether you're using your computers at home, at school, in a small business, or even at a temporary gathering such as a business conference or a camping trip (if you're the sort of person who takes computers along on a camping trip), connecting them through a network makes every one of them more useful and more powerful. And when you connect your network to the Internet, every device on your local network also becomes connected to the Internet.
Note
When you connect two or more computers in a network, each computer becomes more useful. There's a rule that describes this, called Metcalfe's Law. Robert Metcalfe was the original designer of the Ethernet structure used in most modern computer networks; his law states that the value (or power) of a network increases in proportion to the square of the number of devices connected to that network. The math is pretty subjective, but Metcalfe's Law says that two computers connected together are about 4 times as useful as a single computer; if you connect 10 computers, the network is 100 times more powerful, and so forth.
It's not an exaggeration to say that connecting your computers to a network will change your life. Within just a few days or weeks, you will begin to think about everything connected to the network—other computers, printers, game consoles, the Internet, and anything else—as an extension of your own keyboard and monitor. And shortly after that, you will discover new opportunities and services that a network makes possible.
In this chapter, you will learn about the general nature of computer networks and the things you can do with them. You can find more details about using a network later in this book.
Before we begin to consider the things you can do with a computer network, it might be helpful to understand a few basic concepts.
First, the idea of networks is not limited to computers. A network can be any kind of structure that connects individual objects. The highway system is a network, and so is the worldwide telephone system. You can use either of these networks to interact or communicate with other people connected to the same system. Broadcasting networks such as CBS and the BBC use wires, microwave radio links, and other methods to distribute programs from one or more studios to a large number of local stations.
Every network has the following elements in common:
Two or more objects, or nodes, that use the network to connect them
A set of communication channels that carry something—speech, TV shows, computer data—between or among nodes
A set of rules that controls network traffic—on a highway, the rules might specify that vehicles drive on the right and pass on the left, and every car and truck must display a license plate to identify it; in a telephone network, the rules define the form and use of unique numbers (called "telephone numbers") to identify each node and establish connections between them. To assure that a network operates properly, every node and every channel must follow the rules for that particular network.
Next, every network has a maximum carrying capacity. For example, a four-lane Interstate highway can safely carry more cars and trucks at higher speed than a two-lane country road. In a communications network, the capacity of a network connection is the amount of information it can carry, also known as its bandwidth. Both a telephone call and an FM radio station use audio channels, but the same voice sounds better on the FM station because the FM channel has a greater bandwidth that allows more of the original information (in this case, higher audio frequencies) to reach your ear. In a data network, the speed of a network is usually shown in millions of bits (or megabits) per second (Mbps).
And finally, every node on a network has a name. This name might be the same as the name of the person who uses that node, or a description of the location or the type of device at that node. On some networks, the name is a number, or a combination of letters, numbers, and other characters that have no obvious meaning outside of the network (a telephone number is a good example of this type of name). In order to allow the network to accurately find each node, it's essential that every name be unique.