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Suppose there's a Windows Server handling multiple DHCP subnets. The server itself has IP address 10.0.0.1 and is in the subnet 10.0.0.0/24. It has scopes defined for 10.0.1.0/24, 10.0.2.0/24, but NOT for 10.0.0.0/24. If a local DHCP DISCOVER comes in (NOT through relay), what does Windows do? Will it assign an address from 10.0.0.0? 10.0.1.0? 10.0.2.0? Randomly? Ignore it?

Follow up question: If Windows Server ignores the request, and another DHCP server is on the local network segment and has 10.0.0.0/24 defined, will this second server be able to appropriately assign an IP address, and all works well?

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The DHCP server won’t provide addresses on any attached network segment (decided by the IP configuration of the server’s network interface) that doesn’t match the DHCP subnets that are configured. Only a relay can ask for an IP on a different subnet, and the server will respond. This behavior is described in RFC 2131 subsection 4.3.1.

So, in your case, the sever ignores all requests.

If you are trying to use Server 2016 which has support for RFC 3527 and RFC 3011 you actually have to create a dummy address pool on the same network as the relay makes requests from or it still ignores requests for other networks.

If the two Windows DHCP servers detect each other (on the same network segment), one or the other will shut down. Even if they are using different subnets in DHCP. In this case, the server in question would detect the second, working server on the same network its connected to. That behavior can be disabled with:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DHCPServer\Parameters

    Name: DisableRogueDetection
    Type: REG_DWORD
    Data: 0x1

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