Sites are boundaries for intrasite replication. They provide a way to define which domain controller is used for authentication ...

Sites are boundaries for intrasite replication. They provide a way to define which domain controller is used for authentication and directory searches by a collection of client computers on a common IP subnet. Sites are created for IP subnets that contain a large number of users or a small number of users who need to avoid authentication and directory searches across a wide area network (WAN) link. Sites also provide a way for site-aware clients to target authentication requests, directory searches and other operations against a domain controller in the same site or in a nearby site.