First, set a static IP address on your Xbox 360.
Then add the following 3 rules to your router, under the port forwarding options, pointing to the IP address of your Xbox 360:
Port: 88, Protocol: both (tcp/udp)
Port: 2074, Protocol: both (tcp/udp)
Port: 3074, Protocol: both (tcp/udp)
Save those changes, then run the network tests on the Xbox. You should get an “Open” status under that last test, NAT.
Doing this should clear up and “Open Nat” error messages that come up during Halo 3 matchmaking and when joining your friends parties.