An Active Directory domain name that contains one or more labels separated by a dot is referred to as a fully qualified domain name with two or more names, in contrast there is the concept of single-label domain (SLD), which refers to Active Directory domain names with only one label.Given that SLD is not a commonly deployed configuration today.
Not that many Microsoft and third-party applications have not been tested under an SLD configuration as Microsoft recommends FQDN Active Directory deployments and companies who have deployed SLD should transition to an FQDN Active Directory deployment. This will ensure that they get the most value out of their deployed applications.
For companies that will be evaluating transition to FQDN from SLD configurations Microsoft has finally released a whitepaper describing the options and considerations that they will need to take into account. In particular it describes Domain Migration and Domain Rename operations and explains the different considerations of these two options, so that companies can build a transition plan that makes sense to them.
The complete whitepaper “Single-Label-Domains in Active Directory Domain Services (AD DS) – Considerations, Migration, and Co-existence” can be downloaded from http://www.microsoft.com/download/en/details.aspx?id=27143