It is quite possible to boot the same Windows (or Linux) setup on different computers (to boot from CD, to boot from hard disk from another PC, from network share, from terminal server). How are joined-to-AD-domain computers identified/secured/distinguished (SID, machine passwords, GUID, MAC) by Active Directory? Changed later: Changed "Windows" to "OS" in title since I see that Linux machines are also joined to AD as well as configured as AD domain controllers

Joined AD-domain computers are identified by a SID which is a combinaison of the domain SID and a RID (computer SID = domain SID + RID)

How does DC detects if machine is (not) joined to domain? with reference to http://msdn.microsoft.com/en-us/library/dd871382(PROT.10).aspx (3.1.1.2 Computers in Active Directory Domain) tells about MAC or GUID and Marc Russinovich in his "The Machine SID Duplication Myth" (http://blogs.technet.com/b/markrussinovich/archive/2009/11/03/3291024.aspx?wa=wsignin1.0) tells that SID is not used for identification of machines... Machine Account Password Process tells: "As long as no one has disabled or deleted the computer account, nor tried to add a computer with the same name to the domain, (or some other destructive action)" Why cannot computers have the same name if they are identified by whatever else?

If computers are identified by MAC addresses. How is it possible to create an account for an unknown computer in Active Directory and then join it to the domain?

Rather than use (not unique) names, the Windows operating system uses SIDS — Security Identifiers) to identify entities performing actions. These are unique identifiers of alphanumeric security assigned by a domain controller that identify each system , user or object (Group) in a network or on a machine. Some SIDS are identical on all systems. This is a translated paragraph from this link: http://fr.wikipedia.org/wiki/Security_Identifier

Hi, 234051 Unable to Add More Than One User or Object with the Same Name to Active Directory http://support.microsoft.com/default.aspx?scid=kb;EN-US;234051 This posting is provided "AS IS" with no warranties, and confers no rights. Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.