Question on playing two games on same server.

[vjandme]

I have a cable ISP and have a linksys router going
to four computers, two computers are used for gaming. Lan games work just fine and joining two different internet servers works find also, but trying to join the same server one can never get a connection. Is it because both computers are using
is same IP address and if so is there a work around. I realize by being behind a router(hardwarewise) you're behind a firewall, so the outside world sees one IP address where in reality there are four(in my case) computers.
- Thanks in Advance