Active Directory is a distributed directory whereby each Domain Controller (DC) in the forest has a writeable copy of the directory database. As a result, certain operations must be performed on only one DC. Each of these domain controllers is referred to as an operations master or flexible singe-master operation (FSMO).
By default, the first Domain Controller, in the first domain, in the first tree in the forest, holds all five operations master roles and is the only Global Catalog (GC) Server. This should be addressed early in the forest's life by moving some of the OM roles to other DCs and creating more GC Servers, preferably one GC per site.
Active Directory performs a variety of operations that are replicated to