Shriking ranges that are already in use??

Simon Hobson dhcp1 at
Mon Jan 8 18:11:34 UTC 2018

project722 <project722 at> wrote:

> We have a few scopes in DHCP that were made according to inaccurate projections. Now we are being forced to shrink these ranges. 
> For instance, all of our /25's will need to become /27's. We have a legitimate concern that the gateway for the new /27's may already/will probably be leased out from the original /25. 
> What can we do to make this a smooth transition and are there any other considerations that we need to be aware of?

First off, ignore the spaced that will be freed up by the shrinking (for now).

Then in advance, you will want to shorten your max lease time. If your max lease time is (say) 14 days then you cannot guarantee any step is complete until 14 days after you make the change. But if you shorten it, then you will only have to wait out the full time once, and then it will be the shorter time you have to wait. For long lease times, you can balance load/stability vs convenience by a stepped reduction - eg reduce from 14 days to 7 days, wait 7 days, reduce to 3 days, wait 4 days, reduce to 1 day, wait 2 days. Now you will have no leases valid for more than 1 day - you can go shorter if needed.

Make sure that all your routers will have an address in the shrunk ranges.
Eg, if you have used and put the router at then that will not be within But if you've used then that will be OK. If you need to change the router address, add the new one as a secondary address and change the routers option in the DHCP config.

Also change any fixed addresses and all your ranges so that they will fit within the new reduced size subnet.

Wait 1 day (or whatever time you have set) and now all your clients will have suitable addresses and be using the right router address.

You can now change the subnet mask in the DHCP config and wait another day (or time you set). After this, all the clients will be using the new shrunk subnet.

You can now remove the old address from the router (if it changed), and the freed up space is now available to use.

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