Server lists also specify what a client does if it cannot find any preferred servers. A preferred server is any server with a preference of zero, or any server that you have assigned a preference number with nisprefadm.
By default, if a client fails to reach a preferred server, it will then seek out any server it can find anywhere on the network using the search mode described in Default NIS+ Client Search Behavior. You can change this default behavior with the nisprefadm -o option to specify that a client can only use preferred servers and if no servers are available it cannot go to non-preferred servers. See Specifying Preferred-Only Servers in NIS+ for details.
This option is ignored when the machine's domain is not served by any preferred servers.