For advanced operations only.

This optional field specifies a Realm that will be added automatically to the user name of login requests from this NAS if the user name does not already contain a Realm. See the Radiator reference manual for more details.

For example, if you enter open.com.au as the Default Realm, and a user attempts to log in to the NAS as mikem, then the user name will be automatically converted to mikem@open.com.au before being authenticated.