Different internal and external IP addresses?

I can't host a game on gamespy because of different ip addresses in my comp...I heard about a fix to this problem is forwarding ports to blah blah...but I have no clue how to do this, can nyone help me?

www.portforwarding.com will tell you all you need to know. I would help you but I have no idea what router,comp configuration, firewalls, network, etc you have so its nearly impossible to help.