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3.2. How do computers on the Internet talk to one another, or what is TCP/IP?

TCP/IP (which stands for Transmission Control Protocol/Internet Protocol) is the name of a family of more than 100 data communications protocols used to organize computers into networks. The computers that make up the Internet talk to each other in the language of TCP/IP protocols. Any computer that can talk the language of TCP/IP can be a direct part of the Internet. (That's part of the reason why there is such a wide variety of computers on the Net.)

TCP/IP specifies an addressing scheme for computers on the Internet. TCP/IP sets the rules for how data should move between computers and programs on the network. Its protocols are rules that computers must follow in order to move different types of information from place to place. You have heard--or will hear--of some of the protocols that make up TCP/IP, like the File Transfer Protocol (FTP), the Telnet protocol, and the Simple Mail Transfer Protocol (SMTP).

TCP/IP was developed to interconnect systems on ARPANET, PRNET (a packet radio network), and SATNET (a packet-based satellite network). Although all these networks are now defunct, TCP/IP lives.

Messages sent over TCP/IP are called packets. Each packet of information sent over the Internet can be thought of as a letter. TCP/IP puts each letter in an envelope, addresses the envelope with To and From information, and sends the letter on its way. These packets are designed to be small--usually 1500 bytes or so. Most things you send and receive on the Internet (e-mail messages, Usenet postings, files and whatnot) are longer than the maximum packet size, so TCP/IP breaks the message up into packet-sized chunks, addresses each packet, and sends them on their merry way. Once at their destination (actually getting them there is another story) TCP/IP reassembles the packets into one coherent message.

Note: Actually, TCP and IP are two separate protocols that can work in unison. IP moves packets to their destination, whereas TCP checks their integrity and puts them back in their proper order.

Actually getting your message from its source to its destination is fairly painless to understand. The Internet is a store and forward network, meaning that those packets can be sent to (and stored on) any number of computers on their way to their destination. If there is a direct network link between two sites--that is, a physical cable linking the two computers--the packets can zip right over, a nonstop flight with beverage service and an in-flight movie. Most of the time, though, there isn't a direct link. So, the sending computer sends the packets to one that's a little closer to the destination. That machine moves the packets further down the line, and so on, until the packets reach their goal. It's not uncommon for a cross-country message to make 20 or 30 hops. Most of the time, this all happens very, very quickly. Open a Telnet connection from California to New York or Finland and (on a good day) you'll hardly notice any delay at all.

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