Thanks to Wim Verveen for the tip: Microsoft has quietly fixed the 51 IP bug in Windows 2000 that came to light this spring thanks to reports by BugNet and WinInfo. And a WinInfo expose in early April demonstrated that Microsoft knew about and ignored the problem, despite customer complaints, until these press reports brought the issue to attention of the public. But it's been a quiet couple of months for people experiencing the ill side effects of this bug, though Microsoft has finally, quietly released a patch that solves the problem.
The 51 IP bug affects Windows 2000 Server machines that are set up as domain controllers, special systems on a Windows network designed to store and maintain logon information for users and other Active Directory resources. On these systems, it is impossible to add more than 51 IP addresses to the system, regardless of how many network controllers are installed. It's a rare setup to be sure, but Microsoft engineers I've spoken with have acknowledged that the company was aware of the limitation before Windows 2000 went gold and, perhaps more damaging, ignored the problem when customers began complaining in March. After April's reports, however, Microsoft finally began working on a fix.
Because of the infrequency of the bug, Microsoft is requiring customers that need the fix to contact Microsoft Product Support Services (PSS, try the Web site for a local number), though the fix will also be included in the "next" Windows 2000 Service Pack (Whether this refers to SP1 is unknown). However, NTBugtraq has also found a workaround that solves the problem: Simply remove the IP addresses from the pool of IP addresses that DNS can listen on, and configure it to listen on only one IP address. When you add IPs to a system, DNS will normally listen for requests on each of those addresses unless you manually remove them.
For more information about the 51 IP bug, please refer to the Microsoft PSS Web site