Active Directory gives system administrators the option to define sites, consisting of one or more subnets. While it may not be necessary to define sites in all AD deployments, they can come in useful if you have geographically dispersed offices that are separated by slow wide area network (WAN) links. If you have plenty of bandwidth linking offices, so that Active Directory can replicate in a timely fashion, it may not be necessary to create AD sites.
The design of an AD site topology depends on many factors, including network link speed, the number of objects in each domain, and the amount of non-AD traffic flowing over your WAN links. This article is not intended to be a best-practices document, and you should refer to Microsoft TechNet for more information on AD design.
If you have slow network links between offices and need to control AD replication, sites can provide a way to improve reliability. Client PCs will also be able to find a domain controller that is physically close to them for logon. And other applications that rely on AD, such as Exchange Server, can use site topology information to locate services that are close by.
Log in to Windows Server 2012 with a domain administrator account from the forest root domain, or as an Enterprise Administrator.
There’s always one site in AD, which is the default first site if you haven’t created your own sites or renamed the default site.
Assuming you’ve never configured AD sites in your domain before, you’ll see the default first site.
If you expand the site and the Servers folder, you should see a list of the domain controllers in your domain.
Now that you’ve renamed the first default site, it’s time to add more sites.
You’ll see a message telling you to add at least one subnet to the site; and to either install a minimum of one domain controller (DC) in the site or move an existing DC to the new site. Don’t forget you should also have at least one Global Catalog (GC) server in each site.
The DEFAULTIPSITELINK represents the WAN link between my two sites. Depending on the topology of your network and the number of sites, you may need to create additional links. Site links allow you to set the cost, i.e. should the link be used in preference to a slower link, the replication frequency and schedule.
AD sites aren’t much use without subnets, so now let’s add some subnets and assign them to our sites.
You can add more than one subnet to an AD site.