Associating sites and subnets
When you add the Active Directory Domain Services server role to create the first domain controller in a forest, a default site (Default-First-Site-Name) is created in AD DS. As long as this site is the only site in the directory, all domain controllers that you add to the forest are assigned to this site. However, if your forest will have multiple sites, you must create subnets that assign IP addresses to Default-First-Site-Name as well as to all additional sites.
Fonte: https://technet.microsoft.com/en-us/library/cc754697(v=ws.11).aspx