Currently Being ModeratedJan 30, 2013 12:48 PM (in response to lockdown571)
The AirPorts do not support automatic port mapping. You may be thinking of UPnP, which Apple does not use.
Enabling NAT-PMP simply enables the service to operate. You still have to manually setup the ports that you need.
See these User Tips posted by forum expert Tesserax for Port Mapping Basics: