A Primary Domain Controller (PDC) is a server computer in a pre-Windows 2000 NT server Domain.
A domain is a concept used in NT server operating systems whereby a user may be granted access to a number of computer resources with the use of a single username and password combination.
Such domains have at least a Primary Domain Controller, and will often have one or more Backup Domain Controllers (BDCs).
The PDC has the master copy of the user accounts database which it can access and modify. The BDC computers have a copy of this database, but these copies are read-only.
The PDC will replicate its account database to the BDCs on a regular basis. The BDCs exist in order to provide a backup to the PDC, and can also be used to authenticate users logging on to the network. If a PDC should fail, one of the BDCs can then be promoted to take its place.
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