Answer: Windows Server 2008 R2 supports the following Domain Functional Levels:
Windows Server 2003 Mode
When the domain is using this mode you can use domain controllers with the following O/S:
Windows server 2008 R2
Windows Server 2008
Windows Server 2003
Windows 2000
Windows NT
Windows Server 2008 Mode
When the domain is using this mode you can use domain controllers with the following O/S:
Windows server 2008 R2
Windows Server 2008.
Windows Server 2008 R2 Mode
When the domain is using this mode you can use domain controllers with the following O/S:
Windows server 2008 R2.