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FIGURE 5.9 An NTDS network

In an NTDS network, any computer that has Windows NT Server installed can have a copy of the domain database on it. However, only one copy of the database can be considered the master copy. This master copy is the only database responsible for initiating changes on the network. The computer that contains this master copy of the SAM is called the Primary Domain Controller, or PDC. Any other Windows NT server can have a copy of the SAM for backup purposes, in case something happens to the PDC. These computers are called Backup Domain Controllers (BDCs). Further, any computer that has NT Server installed, but doesn’t have a copy of the SAM, is called a Member Server. All of these server types are shown in Figure 5.9.

The importance of these distinctions is that all network administration for a domain happens at a PDC. BDCs can authenticate users, but if the PDC is down, you can’t make administrative changes (such as adding users) until the PDC comes back up. Member servers typically offer services to the network, but don’t take part in authentication (other than to indicate on which server the SAM database resides).

You can promote a BDC to a PDC without reinstalling, but, unfortunately, you can’t promote a Member Server to either a BDC or a PDC without completely reinstalling NT Server.

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