Shriking ranges that are already in use??

project722 project722 at
Mon Jan 8 18:50:40 UTC 2018

Thanks Simon for the very detailed write up. So, if I am understanding you
correctly, lets say we used and set a router address of Next we wanted to shrink that to Will I need to
use a secondary router in the option router line in this case? I'm a bit
confused on this part. When would we need to actually use a secondary
router and what would the option line look like? Something like this:

option routers;

If we do this what determines which router address gets assigned to the
client? ( I'm assuming it can only have one)

On Mon, Jan 8, 2018 at 12:11 PM, Simon Hobson <dhcp1 at>

> 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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