By associating a site with one or more subnets, you assign a set of IP addresses to the site DNS uses the site name to locate a domain controller in that site (or in the next closest site to the client). DNS then provides the IP address of the domain controller to the client for the purpose of connecting to the domain controller. For this reason, it is important to ensure that the IP address that you assign to a domain controller maps to a subnet that is associated with the site of the respective server object.
https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2008-R2-and-
2008/cc754697(v=ws.11)?redirectedfrom=MSDN