Physical Structure
When objects are located in widely distributed geographic areas, or in other cases when slow network connections link separate network subnets, it may be necessary to group computers and printers into separate sites.
Each site may contain one or more IP subnets, and will have one domain controller designated as a bridgehead server. This server is responsible for replicating changes to the global catalog between all local servers, and replicating all site-local changes with bridgehead servers located in other sites. The advantage of this configuration is that replication may be enacted only during off-peak hours, preventing swamping of low-bandwidth connections.