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Using DHCP "Superscopes" to Serve Multiple Logical Subnets

This article was previously published under Q161571
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 2are not capable of assigning addresses from more than one scope to a givenphysical subnet. One workaround for this situation is to add additionalnetwork interface cards (NICs) to the server, and to address each of theNICs to a given logical IP subnet. This involves additional and otherwiseunnecessary hardware, so a new solution was developed and implemented inSP2.

The enhanced DHCP server allows the administrator to create differentscopes (ranges of IP addresses), and then to group those scopes togetherinto 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 Registryvalue, see the following article in the Microsoft Knowledge Base:
ARTICLE-ID: 161429
TITLE : Configuring a Microsoft Windows NT 4.0 DHCP Server for Unicast
multinet multihomed logical subnet scope superscope relay agent super

Article ID: 161571 - Last Review: 11/01/2006 04:58:23 - Revision: 1.1

  • Microsoft Windows NT Server 4.0 Standard Edition
  • kbnetwork KB161571