What allows this federation of networks to cooperate with one another and exchange information is the fact that all are packet switched communications systems based on the TCP/IP protocol standards. TCP (transmission control protocol) and IP (Internet protocol) are two standards that together allow applications to communicate with one another over a wide range of physically different communication systems. Local networks can be based on a variety of technologies, but as long as the software layers on top of the network hardware implements the TCP/IP protocol operations a network can become part of the Internet.
TCP/IP communication depends on addresses being included in each packet. As described previously, routers use these addresses to decide whether to keep a packet in a local network or to forward it to a different network. Addresses have four numeric fields. For example, the address of the machine used by the Computational Science Education Project is 18.104.22.168. To make it easier to remember addresses, there is an equivalent symbolic form, in this case compsci.cas.vanderbilt.edu.
When you are using Internet software to communicate with another site, all you need to know about the site is its name. Most local networks either have a name server or a way to connect to a name server in order to translate symbolic names into internet addresses. If the translation fails, however, you can still make a connection by using the numeric address. For example, suppose you have an account on compsci, and you wish to log in. One way is to use the rlogin program (described in 2.2):
% rlogin compsci.cas.vanderbilt.edu