On Microsoft Servers, a domain controller (DC) is a server computer that responds to security authentication requests (logging in, checking permissions, etc.) within a Windows domain. A domain is a concept introduced in Windows NT whereby a user may be granted access to a number of computer resources with the use of a single username and password combination. On Microsoft Servers, a domain controller (DC) is a server computer that responds to security authentication requests (logging in, checking permissions, etc.) within a Windows domain. A domain is a concept introduced in Windows NT whereby a user may be granted access to a number of computer resources with the use of a single username and password combination. With Windows NT 4 Server, one domain controller per domain was configured as the primary domain controller (PDC); all other domain controllers were backup domain controllers (BDC).