If you’ve ever used a telephone, tracked a package with an overnight shipper, or purchased a new car from a dealership, you’ve used a network. Of course, they weren’t computer networks-they were, respectively, the phone company’s switching network, the overnight shipper’s package-tracking network, and the car manufacturer’s distribution network. And although these networks move phone calls, packages, and cars instead of computer data, they are examples that explain the fundamental purpose of a network. The single most important purpose of any network-computer or otherwise-is to link similar items together using a set of rules that ensures reliable service.

In the telephone network’s case, the rules have to do with what happens when you dial a phone number based on how many digits you dial: If you dial seven digits, it’s a local call; eleven digits is a long-distance call. For the overnight shipper’s network, the rule is that your package is assigned a tracking number that must be recorded each time the package goes through a weigh station or transfer point. And for the car dealership, the rule is that there’s only one reseller within a given geographical area; all new cars are delivered to that dealer, and that dealership has a direct link to the manufacturer. Like other networks, computer networks have basic rules that ensure the safe delivery of information instead of telephone calls, overnight packages, or new cars. A basic set of rules for how a computer network should do its job might look something like this: Read the rest of this entry »