[8FS_Bulau]

If your computer is behind a router, then it has only a private IP address, something like 192.168.x.x
The router will have a public IP address.
When you host a game, and share your public IP address, the router needs to know which private IP to send the packets.

Look up your make and model of router here:

http://www.portforward.com/

for detailed instructions on forwarding a port.
If you can't find exact model, you can probably use something close.