Every domain in an Active Directory forest must contain one of each of the following FSMO roles: * The Relative ID Master allocates security RIDs to DCs to assign to new AD security principals (users, groups or computer objects). It also manages objects moving between domains. * The Infrastructure Master maintains security identifiers, GUIDs, and DNS for objects referenced across domains. Most commonly it updates user and group links.This is another domain-specific role and its purpose is to ensure that cross-domain object references are correctly handled. For example, if you add a user from one domain to a security group from a different domain, the Infrastructure Master makes sure this is done properly. As you can guess however, if your Active Directory deployment has only a single