Shriking ranges that are already in use??

Simon Hobson dhcp1 at
Mon Jan 8 19:24:06 UTC 2018

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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