The best way to link two computers is to use an RJ45 cable to connect the machines' network cards. However, when one or both of the computers has no network card, there is still a fairly easy way to connect them, by using communication ports (found on every PC).
A null modem cable is a 6-conductor shielded cable (meaning 6 wires surrounded by a ground strap), with a serial port connector at both ends. It acts as an inverter cable which matches up the data-sending pins and the data-receiving serial ports of both computers. Technically speaking, it used to link two DTEs without going through two DCEs. The resulting connection, however, cannot be longer than 250 metres.
To create a null modem cable, then, all that is required is to correctly solder the "correct" wires on both ends of the cable. A PC normally has two kinds of ports:
So with these ports free on both computers, there are three possible ways to link them by cable:
|DB9 Number 1||DB9 Number 2|