There are times specific IP address explicitly needs to be added into Active Directory site and services to ensure the application or the servers is talking to specific domain controllers for certain task such as authentication, Global catalog selection etc. Very recent ExRAP program I have worked , indicated some of the Exchange servers were talking to domain controllers out of its own AD SITE. As you may already know Exchange servers will select DC/GC from their own AD Site and if they cannot reach them out they will try to communicate other available domain controller ( DC/GC) the magic lays down on how TCP/IP settings configured on the client as well as AD Site and services , subnets.
In this example we will add Exchange server IP Addresses is 10.10.10.121 /24
On the domain controller we will open Site and Services snap in
Click Subnets , new subnet , type the IP address and Subnet mask and select the corresponding ( desired) AD Site.Once finish you can make right click on Site and make sure the IP address is added on the properties.
By doing this simple task, we made sure the very specific IP address and its subnet mask is added to AD Site we choose.