The core unit of the logical structure in Active Directory is the domain. A domain is a collection of computers defined by an administrator that share a common directory database.
In a Windows 2000 network, the domain serves as a security boundary. The administrator of a domain has the necessary permissions and rights to perform administration within that domain only, unless the administrator is explicitly granted those rights in another domain. Every domain has its own security policies and security relationships with other domains.
Domains are also units of replication. All domain controllers in a domain participate in replication and contain a complete copy of all of the directory information for their domain.
Active Directory uses a multi-master replication model. All of the domain controllers in a particular domain can receive changes to information in Active Directory and replicate those changes to all of the other domain controllers in the domain.
After you install Active Directory and establish a domain, the domain and Active Directory are running in mixed mode. A mixed mode domain supports domain controllers that are running either Windows 2000 or Microsoft Windows NT®. Active Directory installs in mixed mode to provide support for existing domain controllers that have not been upgraded to Windows 2000. You can operate your domain in mixed mode indefinitely, which allows you to upgrade domain controllers running Windows NT on a schedule that meets the needs of your organization.
If your network does not have any domain controllers running Windows NT, or when all of your domain controllers have been upgraded to Windows 2000, you can convert the domain from mixed mode to native mode.
In a native mode domain, all domain controllers run Windows 2000. However, member servers and client computers do not need to be upgraded to Windows 2000 before you convert a domain to native mode. Some Active Directory functionality, such as group nesting and security-type universal groups, requires that the domain be in native mode.
| Caution The change from mixed mode to native mode is a one-way process; you cannot change from native mode to mixed mode. |