The RADIUS feature in Apple airport extreme base station authenticates users via a remote server.
The Access Control list and RADIUS work together. When a user tries to join a network that authenticates using Access Control or a RADIUS server, the base station looks first in the Access Control list, and if the MAC address is there the user can join the network. If the MAC address is not in the Access Control list, the base station checks the RADIUS server for the MAC address. If it is there, the user can join the network.