Shoutbox

Does anybody know how to connect to computer behind a router and has itz own private ip address... when i transfer files with the person and netstat i get the person's ip address assigned by his ISP  but he appears to be behind that address . Any Ideas anyone ?
my friend is using windows xp sp 1...

P.s : I am not gonna use it for malicious purpose or anything just that we can't play online games using direct connect after he set up his lan network... Any help would be appreciated.

Thanks

He needs to configure in a static route or a DMZ into his router to do it.