DHCP support for multiple subnets

Spanning Multiple Subnets in Windows DHCP
Using DHCP to Assign IP Addresses to Secondary Networks
DHCP Requests Fail on Logically Multihomed Server
Using DHCP "Superscopes" to Serve Multiple Logical Subnets

Spanning Multiple Subnets in Windows DHCP

A DHCP (Dynamic Host Configuration Protocol) server can provide IP addresses to clients spanning multiple subnets if the router that separates them can act as an RFC 1542(BOOTP) relay agent. If the router cannot function as relay agent, each subnet that has DHCP clients requires a DHCP server.

You can use a multihomed Windows Server system configured as a DHCP server, with multiple network cards and IP routing enabled, instead of a relay agent for subnetting purposes.

When you have multiple DHCP servers, Microsoft recommends that you place your DHCP servers on different subnets to achieve a degree of fault tolerance, rather than having all the DHCP servers in one subnet. The servers should not have common IP addresses in their scopes (each server should have a unique pool of addresses).

If the DHCP server in the local subnet goes down, requests are relayed to a remote subnet, and the DHCP server there can respond to DHCP requests if it maintains a scope of IP addresses for the requesting subnet. If the remote server has no scope defined for the requesting subnet, it cannot provide IP addresses, even if it has available addresses for other scopes. If each DHCP server has a pool of addresses for each subnet, then it can provide IP addresses for remote clients whose DHCP server is down.

Using DHCP to Assign IP Addresses to Secondary Networks

Some routers can be configured with primary and secondary addresses that allows one port on a router to listen to multiple (logical) subnets.

Windows NT 4.0 Service Pack 2 and later introduced Superscoping support for DHCP. This allows a DHCP server with one network interface card to assign leases for multiple logical subnets.

To setup Superscopes in DHCP Manager, perform the following steps:

1.	Create a new scope in DHCP Manager that corresponds with the primary subnet on the router.
2.	Create the Superscopes that will be a part of the secondary address(es) configured on the router.
3.	Use the DHCP Manager to define the Superscope to include all the subnets.

For complete steps on how to setup DHCP Superscoping, please refer to the following article in the Microsoft Knowledge Base:

161571 (http://support.microsoft.com/kb/161571/EN-US/) Using DHCP Superscopes to Serve Multiple Logical Subnets

DHCP Requests Fail on Logically Multihomed Server

Windows Dynamic Host Configuration Protocol (DHCP) client IP address lease requests fails on a multihomed server configured with multiple logical IP networks on a single network interface card (NIC).

This logical multihomed system is a Windows NT Server with a single NIC configured with multiple IP addresses for one physical segment. If this server is also functioning as a DHCP server with active scopes configured for each logical IP network, DHCP client requests are filled until the scope defining the first logical IP address pool is exhausted. Further DHCP client requests fail to be filled from the pool of IP addresses available from the second or higher logical IP network scopes.

To resolve this problem, install a NIC in the DHCP server for each logical segment. Each NIC should be connected to the same physical segment and assigned only one logical IP address. As DHCP scopes are created they will be made available to the clients via the network interface card at this IP network. Also verify that IP routing has been enabled on this multihomed DHCP server.

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:

ARTICLE-ID: 161429 (http://support.microsoft.com/kb/161429/EN-US/)
TITLE : Configuring a Microsoft Windows NT 4.0 DHCP Server for Unicast

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