A user comment over at MacWindows explains how the AirPort BaseStation can effectively be used as a Cable/DSL router (IP-masquerade style). So, while it looks, to your provider, like you're only using one IP address (that of the BaseStation), you might actually have, say, 5 machines, each with its own IP, sitting behind the AirPort. Naughty, naughty...
Personally, I use this magical...