System Administration Guide: Naming and Directory Services (NIS+)

Remember that all servers are clients of the domain above them, not the domain they serve.

There are two exceptions to this rule:

The root masters and root replicas are clients of the root domain.
NIS+ domain names end with a period. When using a fully qualified name you must end the domain name with a period. If you do not end the domain name with a period, NIS+ assumes it is a partially qualified name. However, the domain name of a machine should not end with a dot in the /etc/defaultdomain file. If you add a dot to a machine's domain name in the /etc/defaultdomain file, you will get Could not bind to server serving domain name error messages and encounter difficulty in connecting to the net on boot up.