Check out
http://www.analogx.com
If you have several machines on a network, but only one connection to the internet. And you want to browse the net from the other machines, just like you can from the machine that's connected. Then what you're looking for is called a Proxy Server, and AnalogX has just what you want. AnalogX Proxy is a small and simple server that allows any other machine on your local network to route it's requests through a central machine. Supports HTTP (web), HTTPS (secure web), POP3 (recieve mail), SMTP (send mail), NNTP (newsgroups), FTP (file transfer), and Socks4/4a and partial Socks5 (no UDP) protocols! It works great with Internet Explorer, Netscape, AOL, AOL Instant Messenger, Microsoft Messenger, and many more!
Best of all it's free last time I checked and I do believe you can specify your IP's internally. Good luck.