In this article we will look at two important elements of the Active Directory infrastructure — sites and subnets. Sites in ADDS are used to combine domain controllers and clients into containers that show the physical topology of your corporate network. Using sites, you can optimize WAN traffic between company branches in different cities/countries. You can bind several TCP/IP subnets to each site, so clients can easily find the Active Directory domain controller closest to them.
An AD site is a set of IP subnets that are connected by a fast network links and are used to control Active Directory replication traffic. Thus, a single site may include several areas of IP addresses. IP subnets are specified in in the network/bitmask format, for example, 192.168.19.0/24.
When you create the first Active Directory domain controller in your domain, a site is created by default named Default-First-Site-Name. The new DC and all the last ones are placed in this site by default.
Suppose your organization has a head office and two branches in different cities. Your task is to create the correct AD site and subnet architecture.
To manage AD sites and subnets, use the Active Directory Sites and Services snap-in (dssite.msc). By default, there is only one Default-First-Site-Name site in the console. Rename it to HQ.
Hint. You can rename the site using PowerShell:
Get-ADReplicationSite Default-First-Site-Name | Rename-ADObject -NewName NewSiteName
Now create 2 new sites:
Click on Sites > New Site.
Specify the site name, select link name (the default is DEFAULTSITELINK with IP transport) and click OK. Create another site.
Now you need to create IP subnets and add them to the appropriate AD site. The list of IP subnets is located in the Subnets section and is empty by default.
Create a new subnet: Subnets -> New Subnet.
Specify the IPv4 subnet and subnet mask in the format 192.168.1.0/24 and bind it to the desired AD site.
Similarly, create all other IP subnets in your organization and map them to Active Directory sites.
You can create a subnet and add it to an AD site using PowerShell:
New-ADReplicationSubnet -Name “192.168.100.0/24” -Site "HQ"
To display all IP subnets, run the command:
Get-ADReplicationSubnet -Filter *
After creating sites and subnets, you can install additional domain controllers in new sites. When you install an additional DC, it will be automatically placed on the site to which the IP subnet of the domain controller is bound. If a site is not assigned to a subnet of a domain controller, by default it will be placed in a site that authorized the promotion of the server to a domain controller.
Site links are used for communication between sites. A site link links 2 or more AD sites and matches the physical connection topology between sites. For example, if all three of your sites can be directly connected to each other, just create a single site link, which includes 3 sites.
You can manage the site with links in the same console in the section Inter-Site Transports > IP. By default, we have only one link named DEFAULTSITELINK with three sites and a replication schedule every 3 hours.