See more AirPort Extreme Base Station radio modes
If you’ve got an AirPort Extreme Base Station—one capable of 802.11n connections—but you’d like to use it on a network without any 802.11n-capable machines, here’s how to limit connections to just non-802.11n machines. Start by launching AirPort Utility (in Applications -> Utilities), select your Base Station in the list on the left, then click Manual Setup.
Switch to the Wireless tab, which is where you control how your Base Station operates. The Radio Mode drop-down is used to determine what types of 802.11 connections your Base Station will support. If you click the drop-down menu, you’ll see four choices—two that accept only 802.11n connections, and two for dealing with older 802.11a, 802.11b, and 802.11g connections.
If you have no 802.11n machines, though, here’s how to completely disable that mode. If you have the Radio Mode drop-down visible, close it now. Next, hold down the Option key, and open the Radio Mode drop-down again. You should now see eight different choices—the four default options, and four options for using just 802.11a, 802.11b, 802.11g, or 802.11b and 802.11g modes. Select the mode that matches the devices you’ll be using, then click Update.
After the Base Station restarts, it will be running in whatever mode you chose.