Microsoft Exchange Server 5.5 sites serve many purposes. Sites establish boundaries for routing: Connectors belong to a site, and Exchange Server 5.5 uses connectors to join sites. Sites serve as centers for directory replication: Objects belong to a site, and Exchange Server replicates objects between sites. Associating objects with sites leads to security boundaries: Administrators set permissions within a site, and these permissions reside in the Exchange Server directory. Finally, you perform administrative work on a site basis: Sites own objects such as servers, connectors, and mailboxes, and only accounts that have the necessary permissions to add, modify, or view the objects can administer the objects.
In Windows 2000, Exchange Server configuration data resides in a set of containers within Active Directory (AD), and administrative groups and routing groups are simply containers waiting to hold other Exchange Server objects (i.e., servers) inside. Exchange 2000 Server uses administrative groups to determine who manages servers and how (e.g., the policies that apply to servers inside each group). Exchange 2000 uses routing groups to define how messages are routed between servers and outside the organization. The combination of administrative and routing groups delivers more flexibility and control than the Exchange Server 5.5 architecture delivers, but you can realize the true power of administrative groups and routing groups only when Exchange 2000 is in native mode. In a mixed-mode organization, legacy Exchange Server machines use a combination of one administrative group and one routing group to represent each site. You can't change this structure until you move the organization to native mode—a one-way operation that you can reverse only by restoring all the organization's servers to the state they were in before the move. (For more information about administrative and routing groups, see "From Sites to Groups," June 2000.)