The Internet is an enormous hierarchy of networks, from a few high speed backbone WANs at the top, down through regional and national WANs, down to smaller WANs and finally to LANs. Only on the LANs are real end-user computers found.

The term internet refers to a collection of separate networks, whether they be WANs or LANs. Internetworking is the study of how to make these networks cooperate in order to provide a seamless image to all the attached hosts that there is but one huge network. This is not all that difficult when all the individual networks and hosts use the same protocols, such as TCP/IP, but if they use different protocol architectures, things get horribly complicated. For example, attaching an IBM SNA network to a DEC DNA network and both of those to the Internet is quite tricky and usually results in lowered performance or less than optimal connectivity. Some hosts may only be able to send mail, while others may be forced to convert all their packets from one form to another in order to send them across an intermediate network.