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Two DHCP servers on the same network segment (different subnets)

Two DHCP servers on the same network segment (different subnets)

Two DHCP servers on the same network segment (different subnets)

Bit of an odd one here....
We are running a single subnet network using DHCP to allocate public IP addresses to all machines (except for a few static servers etc). As an aside, we inherited the public IP address installation. Call it (

This came with an ISP supplied, locked down internet gateway that is the DHCP server. NOTE: it also uses this DHCP scope to  assign addresses to incoming VPN connections.

Now, we changing over to a new ISP,with a sperate firewall/gateway and single public IP address - using internally. In order to gradually move over between the two, both subnets and gateways are on the same network segment - the new gateway's internal interface has two IP addresses on both subnets to ease transition.

Now we are at the stage where we need to set up a DHCP server for the subnet, decommission the DHCP server on the internal segment. HOWEVER, we need to keep the DHCP running for the VPN clients running over the old connection for a few more weeks while we gradually migrate them over to the new gateway.

AS the old DHCP server is externally managed we cannot configure it. I'm loathed to ask them to do it as it's only 9-5 support and I want to do this migration out of office hours (obviously). They've been incompetent in the past (one of the resons we've changed ISP) and I wouldn't really trust them to set this up at 4:50pm then have them go home for the weekend... :)

So what I would like to do is drop a DHCP server on the local segment and for it to have priority over the DHCP server on this segment. I know seperate NT DHCP servers do not co-operate so is there anything I could do to ensure the server fulfils the leases for the VAST majority of clients? maybe some settings to make the new DHCP server respond more quickly?

I maybe able to configure the old DCHP server to some extent as it is accessible remotely via the DHCP admin program but I'm loathed to fiddle around with it too much as it' officially outside of our or responsibilty.


RE: Two DHCP servers on the same network segment (different subnets)

OK, I think I have a simple solution for you. (In theory)
Setup a router/firewall (doesn't need to be big or expensive) between your dhcp assigned public addresses and your new DHCP network segment. set it up with the public DHCP addresses are on the "untrusted" side and the new private addresses ore on the "trusted" side. Set it up to NAT the addresses.

You want all of the internal computers other than the VPN computers to get private addresses from your new DHCP server. No public addresses are being handed out anymore by your old ISP except the VPN which is NATted through the firewall router.

You will need to reboot or realease/renew the DHCP addresses after you make the change, but it doesn't sound like you have that many computers.

hope this helps,

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