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For a more detailed explanation of some features, including their requirements, see What's New in Active Directory Domain Services (AD DS).
To simplify the installation process, ADPREP has been integrated into the AD DS installation and runs automatically as needed.
Error conditions can be corrected to eliminate concerns from a partially complete upgrade.
The wizard also exports a Windows Power Shell script that contains all the options that were specified during the graphical installation.
You can still add a domain controller that runs Windows Server 2012 R2 to an existing domain that has a Windows Server 2003 domain functional level; you just can't create a new domain at that level.
The following table summarizes the new features for AD DS in Windows Server 2012, with a link to more detailed information where it is available.