An Active Directory site definition tells Active Directory to treat the subnets, domain controllers, member servers, and domain-joined device as well-connected. Its Active Directory site link defines the connectivity to other Active Directory sites.
To define connectivity, two sites are needed. The Default-First-Site-Name is present in Active Directory, by default. As Active Directory admins add Active Directory sites, the relationships between the sites evolve to align the logical Active Directory topology with the physical topology of even the most complex environments.
