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A domain controller (DC) is a server that responds to security authentication requests within a Windows Server domain. It is a server on a Microsoft Windows or Windows NT network that is responsible for allowing host access to Windows domain resources.
A domain controller is the centerpiece of the Windows Active Directory service. It authenticates users, stores user account information and enforces security policy for a Windows domain.
A domain controller gives access to another domain in a trust relationship so that a user logging into a domain can access resources in another domain. If the server performing the domain controller role is lost, the domain can still function. If the primary domain controller is not available, the administrator can designate an alternate domain controller to assume the role.
Early versions of Windows such as Windows NT had one domain controller per domain, which was called a primary domain controller. All other domain controllers were backup domain controllers.
Beginning with Windows 2000, the primary domain controller and backup domain controller roles were replaced by Active Directory. The domain controllers in these domains are considered to be equal, as all controllers have full access to the accounts database stored on their machines.