In Mac OS X 10.2 and later, Network preferences lets you choose the "Using DHCP with manual address" option as a way to configure your computer's TCP/IP settings.
If you choose this option when connected to an AirPort or AirPort Extreme base station, you are assigned a subnet mask of 255.0.0.0, and no router address is assigned. You can't connect to the Internet. This could also happen when connecting to any router that doesn't use DHCPINFORM.
To connect to an AirPort base station, configure "Using DHCP" or "Manually".