Mastering Active Directory - Second Edition

Subnets

Subnets represent associated IP address ranges in each site. These are equal to the subnets allocated in network devices, but do not need to be exactly the same. For example, if a site uses 10-20 class C subnets, then instead of adding all these to the AD site's subnet, we can summarize them all into a class B subnet and use them. Based on this subnet information, AD allows objects to locate the closest domain controller. When physical subnets are added or removed, they should be updated in the AD site configuration as well, otherwise, they will pass unmanaged traffic via slow links.