is there a way to add a second domain to an Active Directory installation? We are buying one of our partner companies, and want to migrate their users into our AD into a new domain. Buying another Windows Server to create a new DC is not an option.

Thanks in advance!

  • 1
    I would say migrate to ServerFault, but they rarely answer anything. If this isn't migrated / answered, you might try asking there in a day or so.
    – cutrightjm
    Apr 10, 2012 at 16:56
  • @ekaj No need to post on multiple sites. Users can request migration if no answers are coming. Apr 10, 2012 at 17:12
  • 4
    @ekaj, um, what? You've asked one question, two months ago, with 27 views, which might not even be on topic for the site. If SF was dead I'd understand where you are coming from but as one of the trilogy sites it's far from it.
    – tombull89
    Apr 11, 2012 at 12:42

5 Answers 5


If you're buying the company, I'm assuming you are buying their equipment as well, including their domain controllers. You should be able to set up a two-way forest trust between your domains so users from both companies can access each other's resources. This should be suitable for the time being while you plan a long-term domain migration strategy.


If you're asking if it's possible to have a single server act as a domain controller for two AD domains, then.. no

Depending on the resources of the server, you could look at installing the Hyper-V role and create a new virtual DC for the new domain.


A single Domain Controller cannot provide directory services for more than one domain. You will need at least one additional domain controller. It can be physical or virtual, but it has to exist separately.


You have basically two options here: keep their domain alive and create a trust relationship between it and your own, in order to let the two domains share users and resources, or migrate their users into your Active Directory and get rid of their domain altogether.

The first solution is a lot simpler, but requires keeping their domain around.

The second solution can be quite easy or a real pain, depending on some factors:

  • How many users and computers do you need to migrate?
  • Do they have servers (file, print, database...) to migrate, too?
  • Do they have Exchange in their domain? Do you? If yes to both: do you need to migrate their mailboxes, too?

If the number of users/computers is high and Exchange is involved, you should try avoiding a domain migration, or hire someone who really knows what he's doing.

That said, if you really need to migrate their users and computers into your own Active Directory, you have two choices here, too:

  • Migrate them into your domain (possibly in their own OU hierarchy).
  • Create a new domain into your forest and migrate them into it.

The second option requires at least an additional domain controller (preferably two) for the new domain, because your existing one(s) can not provide domain services for two domains at the same time. Howewer, it is very rarely required to create an additional domain: a proper OU planning and administration delegation can definitely be enough for your needs.


I'd also ask why you're wanting to use a separate domain under the same Forest. Microsoft generally encourages you to accomplish what you want in terms of resource segregation, etc. though OUs and Group Policy.

If you're determined to kill their existing AD, I would look at migrating them into your existing domain, just with proper planning for the OUs.

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