The Active Directory physical topology consists of three main components: site, subnet, and site link

Site#

A collection of machines connected of a LAN. This usually means a physical building. A site usually contains a domain controller, but it doesn't have to. If the site is in an unsecure location, where the physical security of a domain controller can't be assured, the DC might be a Read-only Domain Controller (RODC).

Some services, such as Distributed File System (DFS) and SharePoint are site aware and use the site information-

AD needs at least one site configured. The default site is created when the first domain in your forest is created.

Subnet#

A site link is used to configure which domain controller on a site a user should authenticate against.

The two main functions it serves are the following: - The site links control the AD replication between domain controllers. - Provide a mechanism for a client to discover domain controllers if the DC on site is unavailable.