While a domain is a logical grouping of computers connected for administrative purposes, a site is a physical grouping of computers that are well-connected to one another from the point of view of network bandwidth. Specifically, a site must consist of computers that:
Run the TCP/IP protocol and are located on one or more subnets
Are joined by a high-speed network connection, typically a LAN connection of 10-Mbps or greater with a high available bandwidth, but in some cases slower dedicated WAN connections
Sites are created within Active Directory to mirror the physical layout of a large network. Sites consist of one or more subnets and should mirror the physical connectivity of your network. Computers joined by LAN connections typically form a site, while slower WAN connections form the boundaries between different sites. You thus might have a Vancouver site, a Seattle site, and so on within your enterprise. Sites and domains don’t need to correspond in a one-to-one fashion. For example, one domain may span several sites, or one site may span several domains.
You need to understand a few other concepts to work with sites:
A connection between two sites that involves:
This is a number used to determine which site link will be preferred for replication when two sites are connected by multiple site links. The higher the cost number assigned to a site link, the lower the priority of the link as far as replication is concerned. A cost ...