I actually asked Microsoft's Directory Services team about this question because we store everything in separate attributes for AD (and I am talking ManagedBy, Comments, Notes, blank). Here is Microsoft's response: "The current USN is a 64-bit counter maintained by each Active Directory domain controller as the highestCommittedUsn attribute on rootDSE. Being an unsigned 64-bit integer, that means 264-1, which is 18,446,744,073,709,551,615 (i.e. 18 quintillion). Under normal use that is never going to run out. Even more, when AD reaches that top number, it would restart at 1 all over again!" You can read about the ManagedBy and storing the last logged on user here: Find Out What Computer a User Logged Into - DeployHappiness. You can read Microsoft's full reply here: Friday Mail Sack: Drop the dope, hippy! edition - Ask the Directory Services Team - Site Home - TechNet Blogs
