To provide smooth and trouble-free migration, next Generation Windows NT Directory Services were designed to operate in a mixed environment. A mixed domain, with both next generation and "down-level" Windows NT 3.5x and 4.0 domain controllers, works and acts just like a Windows NT 4.0 domain. A tree cannot be formed with a mixed domain, because the down-level domain controllers don't understand trees or Kerberos authentication.
The migration process from down-level servers to next Generation Windows NT Directory Services can take place one domain controller at a time, and when all of the domain controllers in the Windows NT 4.x domain have been upgraded, it can be switched to a next generation domain. Therefore, the best way to prepare for next Generation Windows NT Directory Services is to install Windows NT 4.0 today. To see how this would work, see the following section.