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Moving Bitsfrom One Place to Another Modern networking is built around the concept of "layers of service." You start out trying to move bits from here to there, losing some along the way. This level consists of wires and hardware, and not necessarily very good wires. Then you add a layer of basic software to shield yourself from the problems of hardware. You add another layer of software to give the basic software some desirable features. You continue to add functionality and smarts to the network, one layer at a time, until you have something that's friendly and useful. Well, let's start at the bottom and work our way up. Packet Switch Networks When you try to imagine what the Internet is and how it operates, it is natural to think of the telephone system. After all, they are both electronic, they both let you open a connection and transfer information, and the Internet is primarily composed of dedicated telephone lines. Unfortunately, this is the wrong picture, and causes many misunderstandings about how the Internet operates. The telephone network is what is known as a circuit switched network. When you make a call, you get a piece of the network dedicated to you. Even if you aren't using it (for example, if you are put on hold), your piece of the network is unavailable to others wishing to do real work. This leads to underutilization of a very expensive resource, the network. A better model for the Internet, which may not instill confidence in you, is the U.S. Postal Service. The Postal Service is a packet switched network. You have no dedicated piece of the network. What you want to send is mixed together with everyone else's stuff, put in a pipe, transferred to another Post Office, and sorted out again. Although the technologies are completely different, the Postal Service is a surprisingly accurate analogy; we'll continue to use it throughout this chapter.