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I have a device which acts as a router and DHCP server. I'm replacing it, and would like to minimize down time.

  1. If I unplug it, and plug in a different device with the same IP, will all the PCs with DHCP leases keep on working? (I have DHCP Conflict detect on, so it shouldn't reassign a DHCP address already used).

  2. What if I want to change the IP (new subnet) - is there anyway to tell all the clients (Windows PCs) to release their DHCP leases and request new ones in a minute?

  3. If before unplugging the old device, I have it release all DHCP leases, will the Windows PCs automatically ask for new DHCP addresses?

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DCHP lease are client based. It's the client that request lease, ask later if it can renew it (to extend it and so keep the ip).

While the lease is valid, clients keep their config. But they will contact again the dhcp server before the end to try to renew it.

So you could have a dhcp lease big enough to handle the change. Take care, removing the router will make client unable to reach what was behing the router (internet for example), while the new one is not ready.

If the new router provide ip using another subnet, client will use this new subnet when their current lease expire. To handle the change, your router must have ip in both new and old (2 ip on same ethernet card), and provide lease only on the new one. So old client still works, and new one too.

You could do a batch using psexec for example to connect remotely on each computer and do an ipconfig /renew to force the change before lease expire.

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Good idea for psexec and second subnet on router. +1 for you. – user3914 Jan 16 '11 at 19:19

The DHCP should continue to work until the leases expire, since once a machine has an IP address it will try to hold onto it unless told otherwise.

So to answer your questions:

1) Yes.

2) Not if you want to change the subnet, no.

3) Yes, probably. I say probably, but the DNS might be cached so you may need to run ipconfig /flushdns and ipconfig /registerdns after the fact.

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Thanks, Randolph. Can you document a source for #3? I'm not worried about DNS caching because DNS is handled by a different machine and staying the same. – SRobertJames Jan 16 '11 at 5:45
My source is the old "experience" talking, I'm afraid. Windows 7 and Vista can happily acquire new DHCP settings without blinking, but XP sometimes needs a "repair" on the network icon. It's random. – user3914 Jan 16 '11 at 5:51
I would also point out that if you replace the router with another device, the MAC address will obviously change and devices may have to re-arp for the MAC address of the gateway. – SpacemanSpiff Jan 16 '11 at 6:48
Tom - Will they automatically re-ARP, or will there be a down period which they don't get an answer but don't try reARPing? – SRobertJames Jan 16 '11 at 7:22
Changing IP address while a connection is open will break the connection. Some applications can handle this better than others. Worst case the request will end up partially completed. (i.e. Credit card was charged for the flight but the tickets weren't issued.) – BillThor Jan 16 '11 at 7:22

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