Next, you need to decide whether you plan to integrate Windows 2000 DNS with a legacy DNS infrastructure and whether you need to consider the impact of an Internet presence for your corporate DNS name space design. During the DNS design, remember that this is a critical service for Active Directory and Windows 2000. The DNS server infrastructure must be fault-tolerant, highly available, easily accessible and must provide minimal latency for the replication of DNS database changes. As with Windows NT, a domain in Windows 2000 is a security boundary, but it's also an Active Directory partition. These domain structures are called trees and forests .
Trees are hierarchies of domains linked by trust relationships.
Each tree shares a contiguous name space. A forest is a set of trees linked together via trust relationships. The trees that are joined together at the top level of the forest don't necessarily have to share the same name space. During your domain model design, you'll have to decide upon this domain structure, how many domains you want, how they'll fit together in trust relationships and how they'll be linked to your namespace design. When designing your OU hierarchy, you'll probably want to start off by examining your organization's administrative support hierarchy.