Shriking ranges that are already in use??

project722 project722 at
Mon Jan 8 20:30:10 UTC 2018

OK I'm with you now. I think we'll be OK. These are fairly new subnets so
not that many clients on them to break :-).

Thanks again for the help.

On Mon, Jan 8, 2018 at 1:24 PM, Simon Hobson <dhcp1 at> wrote:

> On 8 Jan 2018, at 18:50, project722 <project722 at> wrote:
> > 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?
> No, because is a valid address in the subnet.
> Had you used (say) then that would NOT be valid in the
> subnet - and so you'd need to change it. Since you can't
> "just change it" without breaking clients for a while, it's best to add the
> new address as a secondary address on the router* so that clients with old
> leases will carry on working until they get new settings via DHCP.
> * Ah, reading it over I see the confusion. It's not about adding a second
> router address in the DHCP config, it's about adding it on the router
> itself. On a Linux box something like :
> > ip addr add dev eth0
> on a Cisco box something like :
> > int gi0/0
> > ip address secondary
> That way, clients still configured to use will continue
> working, when things get their new config then they'll switch to using
> When everything has been reconfigured, you can remove the
> address from the router and make the primary (and
> probably, only) IPv4 address.
> I did miss one step BTW - when all the clients have been reconfigured, the
> router will need reconfiguring - if it originally had then
> it will need changing to
> Reconfiguring the network is never something you could call fun - I've
> done it a couple of times :-( Assume that you'll have missed something - a
> device you didn't realise was manually configured, a device configured to
> talk to a PC at a particular address (common when third parties install
> things like access control systems or multi-funcion copier/scanners), and
> so on.
> Given the number of outfits I've come across who just still don't
> understand IPv4 basics, I hate to think how they'll cope with IPv6 8-0
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