If you are using the Lion or Mountain Lion operating system on your Mac.....
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Exrreme icon, then click Edit
Click the Network tab at the top of the window
Change the setting for Router Mode from DHCP and NAT to "Off (Bridge Mode)"
Click the Update button at the lower right of the window and the AirPort will restart in 30 seconds
Then, power off your complete network, wait a minute, start the modem first and let it run a minute, then start the AirPort next the same way, then start other devices one at a time the same way.
If you are using the Leopard or Snow Leopard operating system on your Mac.......
Open Macintosh HD > Applications > Utilities > AirPort Utilty
Click on the AirPort Extreme, then click Manual Setup
Click the Internet icon
Click the Internet Connection tab
Change the setting for Connection Sharing to "Off (Bridge Mode)"
Click Update at the lower right of the window and the AirPort will restart in 30 seconds.
Then, power cycle the entire network as instructed above