when playing multiplayer,my NAT is STRICT,which i have no idea what that is but my other games like CODMW2 and BLACK OPPS my NAT is OPEN?? any one know how i can make my NAT open in CODMW3? please?????
Did you remember to power off / power on your router?
If it's still now working try some of the MW3 forums and see what others with the same router are doing to adjust the security settings on the router.