Disjoint namespace is a scenario in which a computer's primary domain name system (DNS) suffix doesn’t match the DNS domain name where that computer resides. The word disjoint is a synonym for separate. The computer with the suffix that doesn't match is called the disjoint.
In Active Directory, domain member computers may be able to function in a disjoint namespace with support in certain situations. However, network applications may not work because the applications assume the namespace is identical to the DNS suffix of all domain member computers.Content Continues Below
Disjoint namespace can occur in Windows Server 2003 and later versions.