If you have a active directory domain that has a single labelled DNS suffix ie its called "domain" instead of "domain.com" or "domain.local" and you are trying to either add an additional domain controller to the single labelled domain, or create a child or tree domain to a single labelled forest root domain you need to include this registry key value on every DC in every domain.
1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following subkey:
3. In the right pane, locate the AllowSingleLabelDnsDomain entry. If the AllowSingleLabelDnsDomain entry does not exist, follow these steps:
a. On the Edit menu, point to New, and then click DWORD Value.
b. Type AllowSingleLabelDnsDomain as the entry name, and then press ENTER.
4. Double-click the AllowSingleLabelDnsDomain entry.
5. In the Value data box, type 1, and then click OK
6. Quit Registry Editor.
If you dont you will not be able to join the domain controller to the domain or to your new child/tree domain and you will recieve this error: