Many organizations can function adequately with a single ADT as domain.
However some organizations have requirements that necessitate the deployment of multiple domains.
These requirements can include domain replication requirements in some cases organizations have several
large offices that are connected by slow or unreliable white area networks the network connections might
not have enough bandwidth to support a tedious replication of the domain partition in such cases installing
a separate Active Directory Domain in each office might be better.
Another requirement is DNS namespace requirement.
Some organizations require more than one DNS namespace in an 80 days forest.
Typically this is the case when one company requires another company or merges with another organization
and preserving the domain names from the existing in wire amount is necessary.
It is possible to provide multiple user principal names for users in a single domain but many organizations
choose to deploy multiple domains in this scenario.
Please know that deploying separate domains provide administrative autonomy but not administrative isolation.
The only way to ensure administrative isolation is to deploy a separate forest.
Another requirement can be forest administrative group security requirements.
Some organizations might choose to deploy a dedicated or empty road domain.
This is a demand that doesn't have any user accounts other than the default forest route.
Domain accounts they aided is forest through domain has two groups the schema Edmonds group and the
enterprise admins group that do not exist in any other domain.
In the ADA the US Forest because these groups have far reaching rights in an 80 year forest.
You might decide to restrict the use of these groups by using only the ADA the US Forest route domain
to store them.
Another requirement is reserved domain requirements.
Some organizations deploy resource demands to deploy specific applications.
With this deployment all user accounts are in one domain whereas application servers and application
administration accounts are deployed in a separate domain.
This enables application administrators to have complete domain administrative permissions in the resource
domain without enabling any permissions in the demand that contains the regular user records.
And last but not least distributed administration requirements.
Organizations might have corporate security or political requirements to employ a distributed administration
model.
Organizations can achieve administrative autonomy by deploying a separate domain with this deployment
domain administrators have complete control over their domains and of course always remember to choose
the simplest design that achieves the required goal.
It's less costly to implement and more straightforward to administer.