Which functional level only allows Windows Server 2003 and Windows 2008 domain controllers?

The functional level that only allows Windows Server 2003 and Windows Server 2008 domain controllers is the Windows Server 2003 functional level. At this level, the Active Directory domain can utilize features introduced in Windows Server 2003 while maintaining compatibility with the earlier Windows 2000 domain controllers. However, it does not support any domain controllers running versions newer than 2008.