Using DHCP "Superscopes" to Serve Multiple Logical Subnets

Windows NT 4.0 Service Pack 2 (SP2) provides support for a new DHCP "superscope" feature. This feature allows a Windows NT DHCP server to:

• Support DHCP clients on locally attached networks that have multiple logical subnets on one physical network (sometimes referred to as a "multi-net").
• Support DHCP clients on the far side of bootp relay agents, where the network on the far side of the relay agent has multiple logical subnets on one physical network.

Versions of Windows NT DHCP server prior to Windows NT 4.0 Service Pack 2 are not capable of assigning addresses from more than one scope to a given physical subnet. One workaround for this situation is to add additional network interface cards (NICs) to the server, and to address each of the NICs to a given logical IP subnet. This involves additional and otherwise unnecessary hardware, so a new solution was developed and implemented in SP2.

The enhanced DHCP server allows the administrator to create different scopes (ranges of IP addresses), and then to group those scopes together into a superscope.

To create a superscope, complete the following steps:

1. Create each of the scopes using DHCP Manager. Assign global and scope properties as desired. Be sure to enable each scope.
2. In DHCP Manager, select the DHCP server. Click Scope, and then click Superscopes.
3. Click Create Superscope, supply a name for the superscope, and then click OK.
4. Add the appropriate scopes from the Available Scopes list to the Child Sub-Scopes list. When finished, the Child Scopes should be listed from top to bottom in the order that you want addresses to be used from them.
   
   NOTE: The order that you add the Child Sub-Scopes is of no consequence. DHCP Manager will sort them in ascending order.
5. Click OK.
6. If the DHCP server is configured with the IgnorebroadcastFlag DHCP Registry value set to 0, and if the Superscope is on a subnet that is directly attached to the server (that is, not being reached via a DHCP relay agent), then each of the logical subnets must be directly reachable by the DHCP server. This means that there must be a local route to each logical subnet. The easiest way to accomplish this is to add an IP address to the NIC on the local server for each of the logical subnets that it is attached to.

For additional information about the IgnorebroadcastFlag DHCP Registry value, see the following article in the Microsoft Knowledge Base: