Before you can migrate Windows NT 3.5 or NT 3.1 machines to Windows 2000 (Win2K), you must first migrate them to NT 4.0 or NT 3.51, respectively. You can migrate NT 4.0 and NT 3.51 machines directly to Win2K.
You must migrate a domain's PDC before you migrate any other domain controllers. After you migrate the PDC, the users and groups from the domain's directory database load into Win2K's Active Directory (AD). Win2K considers all domain controllers equal. After you've migrated the PDC, it's no longer a PDC because Win2K makes that designation obsolete.
The migration process requires that you specify where to place your domain in the AD hierarchy. You have three choices for designating your domain: as a child of an existing tree of domains, as the root of a new tree of domains in an existing forest, or as the root of a new tree in a new forest. After you migrate the PDC, Win2K considers your domain a mixed domain because NT 4.0 or NT 3.51 domain controllers are still present. You can use the Win2K administration tools to manage the mixed domain. All the NT machines—BDCs, member servers, and clients—within the domain remain unaware that you've upgraded the PDC to an AD server, and those machines will continue to function without interruption.
As you upgrade each BDC to Win2K, the upgrade process recognizes the BDC and automatically installs it as an AD replica within the replication topology. After you upgrade all the BDCs to Win2K, the domain is no longer a mixed domain. For more detailed instructions about migrating your existing domain models to Win2K, see Sakari Kouti, "Migrating to Active Directory," January 1999.