A tree in Windows 2000 simply signifies a DNS namespace. More simply, an Active Directory tree is a collection of domains. It consists of a single-tree root domain with subdomains completing the Active Directory hierarchy (see Figure 3.6).
There are two types of namespaces that you need to consider when implementing Active Directory.
ContiguousA namespace in which each level of the domain hierarchy is directly related to the levels above and below it. For example, the tree root for Help and Learn, Inc. is helpandlearn.com. The subdomains are named east.helpandlearn.com and west.helpandlearn.com. As you can see, all domains share the root helpandlearn.com domain name.
DiscontiguousA namespace that is based on different root DNS domain names. For example, microsoft.com and helpandlearn.com are two separate (that is, discontiguous) namespaces. A discontiguous namespace is used with multiple trees in a forest.
The namespace for an Active Directory tree is contiguous, because each domain in the hierarchy is directly related to the domains above and below it in the tree.
Figure 3.6 The hierarchy of an Active Directory tree.
Like DNS, Active Directory trees are represented as inverted trees. Trust relationships between the domains in the tree are created automatically in Windows 2000, unlike Windows NT, which requires all trust relationships to be created manually.
An Active Directory tree is created automatically when a new root domain is installed.