When your AirPort Extreme Base Station is configured to share your Internet connection with other machines, the NAT capabilities let the Base Station serve private network addresses to all the computers on the net work.
The AirPort Extreme Base Station is a great product with few limitations.
US$249 for Airport Extreme Base Station (with modem and antenna port)
The new 2TB Time Capsule features full airport extreme base station
with 802.11n Wi-fi technology, weighing 1.6 kg and 1.4 inches height, a frequency band of 2.4GHz and 5 GHz simultaneously and it has secure Wi-Fi Protected access, NAT firewall, support for RADIUS authentication and time-based access control.