The most common program which is required in networking is the one which connects the PC on a persons desk to the main computer. The Kermit program is the most freely available program which provides such connectivity along with protocols for file transfer. It also provides dialling features where required.
The next form of connectivity is the dial up link over which a computer-to-computer transaction takes place--including transfer of mail, files and so on. Between Unix-like machines one uses UUCP or Unix-to-Unix-copy. A most general purpose suite of such programs is part of the GNU project and is called Taylor UUCP. It can ``talk'' to a number of other UUCP's. For the PC we have a public domain utility called UUPC which is actually a Unix-to-PC copy.
The quickest form of connectivity is the Internet. This is usually used via the TCP/IP protocols. Most often the IP layer is implemented within the Operating System. A suite of programs required to administer and use the Internet are now in the Public Domain as part of the BSD/NET release. One can find many of these programs operating in the Linux environment.