Shriking ranges that are already in use??

[Simon Hobson]

On 8 Jan 2018, at 18:50, project722 <project722 at gmail.com> wrote:

> Thanks Simon for the very detailed write up. So, if I am understanding you correctly, lets say we used 172.16.0.0/25 and set a router address of 172.16.0.1. Next we wanted to shrink that to 172.16.0.0/27. Will I need to use a secondary router in the option router line in this case?

No, because 172.16.0.1 is a valid address in the 172.16.0.0/27 subnet.

Had you used (say) 172.16.1.254 then that would NOT be valid in the 172.16.0.0/27 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 172.16.0.1/25 dev eth0

on a Cisco box something like :
> int gi0/0
> ip address 172.16.0.1 255.255.255.128 secondary

That way, clients still configured to use 172.16.1.254 will continue working, when things get their new config then they'll switch to using 172.16.0.1. When everything has been reconfigured, you can remove the 172.16.1.254 address from the router and make 172.16.0.1 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 172.16.0.1/25 then it will need changing to 172.16.0.1/27.


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