Firewall trouble with halo2 on xbc and gamespy

I have a Sprint 645 series dsl modem/router. I cannot host games on xbc due to the fact that the modems router properties are causing me to transmit an Ip that obviously is not my real IP. I know how to go into the modems settings by using the telnet command. However, if I try to turn off IP routing, my connection to the internet fails.

Please help me!

 

Try port forwarding the poper ports to the xbox ip inside the router.