The first version of Active Directory with Windows 2000 was surprisingly stable and robust. Microsoft does not have the best track record for initial releases of products, but they must be commended for Windows 2000 Active Directory in terms of its feature rich-ness and reliability. That said, since Active Directory is such a complex and broad technology, there was still much room for improvement. There were some issues with scalability, such as the infamous 5,000-member limit with groups or the 300-site limit, which may have imposed artificial limitations on how you implemented Active Directory. Both of these issues have been resolved in Windows Server 2003. The default security setup with Windows 2000 Active Directory out-of-the-box was not as secure as it should have been. Signed LDAP traffic and other security enhancements have since been added into service packs, but they are provided by default with Windows Server 2003. Finally, manageability was another area that needed work in Active Directory, and in Windows Server 2003 numerous command-line utilities have been added along with some significant improvements to the AD Administrative snap-ins.
We have highlighted a few key areas where Active Directory has been improved in Windows Server 2003, and we'll describe more new features in the next section. If you already have a Windows 2000 Active Directory infrastructure deployed, your next big decision will be whether and when to upgrade to Windows Server 2003. Fortunately, the transition to Windows Server 2003 is evolutionary, not revolutionary, as with the migration from Windows NT to Active Directory. In fact, Microsoft's goal was to make the move to Windows Server 2003 as seamless as possible, and for the most part they have accomplished this. You can introduce Windows Server 2003 domain controllers at any rate you wish into your existing Active Directory environment; they are fully compatible with Windows 2000 domain controllers.
Before you can introduce Windows Server 2003 domain controllers, you must prepare the forest and domains with the ADPrep utility, which primes the forest for new features that will be available once you raise the functional level of the domain or forest. Functional levels are similar in nature to domain modes in Windows 2000 Active Directory. They allow you to configure different levels of functionality that will be available in the domain or forest based on which operating systems are running on the domain controllers.
Before we cover the upgrade process to Windows Server 2003, we'll first discuss some of the major new features in Windows Server 2003 and some of the functionality differences with Windows 2000. Based on this information, you should be able to prioritize the importance of how quickly you should start migrating.