A domain controller is a computer running Windows 2000 Server that stores a replica of the directory. A domain controller also manages the changes to directory information and replicates those changes to other domain controllers in the same domain. Domain controllers store directory data and manage user logon processes, authentication, and directory searches.
A domain can have one or more domain controllers. A small organization that uses a single local area network (LAN) may need only one domain with two domain controllers to provide adequate availability and fault tolerance, while a large company with many geographical locations will need one or more domain controllers in each location to provide adequate availability and fault tolerance.
Active Directory uses multi-master replication, in which no single domain controller is the master domain controller. Instead, all of the domain controllers running Windows 2000 Server within a domain contain a writeable copy of the directory. However, domain controllers might hold different information for short periods of time until all of the domain controllers have synchronized their changes to Active Directory.