familurize yourself with network type and their protocols. Find libraries that work with tcp/ip etc. Learn how to access the modem and read/write from it, or the comm/serial ports. If writing your own libraries, make sure you can work with some kind of ip address (for a intranet). if your trying to create something like a generic mainframe, you might want to familurize yourself with the insides of virtual operating systems and multitasking.
basically read/write from one computers ports (modem...) into/from another computers ports. program might want to know how much data to send at a time, how much is being recieved, where to route data to, where data is from. encription/decrytpion. read up on windows sockets and basicall create someform of working copy. use non-windows sockets. unix,linux...
i hope i help and answered the question the way to where looking for.