Remote Authentication Dial-In User Service (RADIUS) is a client/server protocol that allows remote access servers to authenticate wireless users against a central database, eliminating the need to maintain user information on each device in the network.
After the RADIUS server is set up (see the documentation that came with the server), open AirPort Utility, select your wireless device, and choose Base Station > Manual Setup. Enter the password if necessary. Click AirPort in the toolbar, and then click Access Control.
AirPort Utility is in the Utilities folder in the Applications folder on a computer using Mac OS X, and in Start > All Programs > AirPort on a Windows computer.
Choose RADIUS from the MAC Address Access Control pop-up menu.
Choose Default or Alternate from the "RADIUS type" pop-up menu, depending on how the RADIUS server is set up.
Enter the IP address and other information for the RADIUS server. Check with the RADIUS server administrator if you’re not sure what information to enter.