Airport has its own internal NAT firewall as does any wireless device.

1. Apple Airport Extreme/Express for Mac users * (NAT-PMP)

else

2. Port Forwarding for the Apple AirPort Extreme Base Station but don't forget to set up a Static ip first or else the process won't work. Static ip for OSX, choice of instructions ... PortForward.com - Free Help Setting up Your Router or Firewall OR Computing & Information Technology : University of Vermont OR World of Warcraft Community Site -> Support OR OSX for Static IP (Sonic.net, Inc.)

3. OSX Firewall settings for Tiger