DHCP Reservations Do Not Get Assigned from a Superscope

• Microsoft Windows NT Server version 4.0

SYMPTOMS

When you are using a dynamic host configuration protocol (DHCP) superscope, reservations made in any subscope, except for the first subscope, may not be given out.

For example, you have a single segment using the following superscopes:

192.168.1.1-255, subnet mask 255.255.255.0 192.168.2.1-255, subnet mask 255.255.255.0

You use one DHCP server, create a scope for each network, and then create a superscope using these two scopes. This allows your DHCP server to assign addresses from either scope to clients on the same physical segment.

Then you create a reservation in scope #2 for a client computer. When the client computer tries to get an address using DHCP, the DHCP server gives the client computer an address from the first scope instead of giving it the reservation in the second scope.

CAUSE

This is happening because a DHCP server normally hands out addresses from a superscope starting with the first scope. After that scope is full, the server starts handing out addresses from the second scope, third scope, and so on.

Because the reservation is in the second scope, the DHCP server gives out an address from the first scope if it is not full. If the first scope had been full, then the client reservation would have been given out correctly.

STATUS

Microsoft has confirmed this to be a problem in Windows NT 4.0 Service Pack 2. This problem has been corrected in Microsoft Windows NT 4.0 Service Pack 3. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K

   ARTICLE-ID: Q161571
   TITLE     : Using DHCP "Superscopes" to Serve Multiple Logical Subnets

Keywords          : NTSrv nttcp kbnetwork kbbuglist
Version           : WinNT:4.0
Platform          : winnt
Issue type        : kbbug
Solution Type     : kbservicepack