Realms
Realms are identifiers that are placed before or after the values normally contained in the User-Name attribute that a RADIUS server can use to identify the server to contact to start the authentication and accounting process.
The following example illustrates how realms are used with RADIUS:
User, Joe, is employed by company XYZ in Sacramento. The realm for this area is SAC. However, Joe is currently in New York City on a remote assignment. The realm for New York City is NYC. When Joe dials into the NYC realm, the User-Name passed is SAC/Joe. This notifies the NYC RADIUS realm server that this packet needs to be forwarded to the server that does the authenticating and accounting for SAC realm users.