Understanding domain trees and forests
Each domain in the directory is identified by a DNS domain name and requires one or more domain controllers. If your network requires more than one domain, you can easily create create multiple domains.
Understanding Forest/Trees
One or more domains that share a common schema and global catalog are referred to as a forest. If multiple domains in the forest have contiguous DNS domain names, as shown in the first illustration, then that structure is referred to as a domain tree. If, as shown in the second illustration, multiple domains have noncontiguous DNS domain names, then they form separate domain trees within the forest. A forest can contain one or more domain trees. The first domain in a forest is referred to as the forest root domain.
You create a domain by installing the first domain controller for a domain. During installation of the first domain controller, the Active Directory Installation wizard uses information you provide to install the domain controller and create the domain within the existing context (if any) of relationships to other domains and domain controllers. This context may be the first domain in a new forest, the first domain in a new domain tree, or a child domain of an existing domain tree.
After you install the first domain controller for a domain, you can install additional domain controllers in an existing domain for fault tolerance and high availability of the directory.
Global catalog and infrastructure master
The infrastructure master is responsible for updating references from objects in its domain to objects in other domains. The infrastructure master compares its data with that of a global catalog. Global catalogs receive regular updates for objects in all domains through replication, so the global catalog's data will always be up-to-date. If the infrastructure master finds data that is out-of-date, it requests the updated data from a global catalog. The infrastructure master then replicates that updated data to the other domain controllers in the domain.
Important
Unless there is only one domain controller in the domain, the infrastructure master role should not be assigned to the domain controller that is hosting the global catalog. If the infrastructure master and global catalog are on the same domain controller, the infrastructure master will not function. The infrastructure master will never find data that is out of date, so will never replicate any changes to the other domain controllers in the domain.
If all of the domain controllers in a domain are also hosting the global catalog, all of the domain controllers will have the current data and it does not matter which domain controller holds the infrastructure master role
