Most routers have built-in software for Network Address Translation (mentioned by a poster above). Such software is mostly administration oriented. It will give you your real IP address, and open/route specific traffic to an IP address, etc.
You can usually access your software by typing
http://192.168.0.1 in your browser. You'll then have to login with a username/password (this is where your user manual is important, because chances are you haven't actually changed the default settings). The manual will usually tell you the default username/password.
After logging in, there'll be a place to see your real IP address and assign ports to a specific local address (i.e. 192.168.0.100).
That's the process in a nutshell...my roommates and all share one Cable connection with six computers using a router and switch. I was able to program the router and setup FTP, HTTP sites even with my computer behind the switch and router.