Active Directory sites govern access and replication.
Active Directory's DClocator process allows for devices to find the nearest domain controller. By default, these would be the domain controllers in the current Active Directory site where the device resides. The way the device knows in which site it resides is derived from its IP address, which matches a subnet, as defined for the Active Directory site.
When there are no domain controllers in an Active Directory site, the site link costs define the nearest domain controller to DClocator. The domain controllers in the site connected with the lowest cost will be returned to the device.
Domain controllers in different Active Directory sites replicate partitions over the same Active Directory site links through bridgehead servers; these are domain controllers that take on the additional role...
