Design security Windows 2000 provides for easier security management and stronger security features, but they also add complexity to your overall Active Directory design. The Windows 2000 PKI can be tightly integrated with Active Directory and thus leverage the effort and cost you put in your Active Directory design. The Windows 2000 site topology is a logical model, layered on top of the physical network that must accurately reflect the underlying network topology. In Windows 2000, the site topology sits above the physical network layer and below the domain structure layer. It's important to make a distinction between domain structure design and site topology design.
Sites reflect the location of user communities, whereas domains contain objects.
A domain is mapped to a site by placing a replica of the domain within the site. A site doesn't contain a portion of a domain. It contains the entire domain. Sites contain domain controllers are entire replicas of a domain. If a GC is placed in a site, then the entire Active Directory becomes available to the site.