Best practice on removing IP Scopes(subnets)

Simon Hobson dhcp1 at
Fri Apr 24 15:22:35 UTC 2009

Jeremy Phillips wrote:

>   We are currently in the process of moving our IP space.  I'm 
>trying to remove the old IP subnets from the dhcp server without 
>Customers or having our helpdesk have 1000's of calls from customers 
>that need to get a new IP.  Essentially we will be moving 20+ class 
>I was just wondering how some of you went about this very procedure 
>without effecting service.

<pedant>I believe the word you are after is affect, not effect. I 
believe you wish to continue effecting service.</pedant>

How you go about the change depends on how urgent it is, and what 
your equipment capabilities are.

The most graceful way is to simply stop handing out addresses*, 
configure the routers with shared subnets (new+old on same 
interface), and wait. Over time the clients will migrate to the new 
subnet as their leases expire. Many clients will get a new address at 
boot time (or more technically correct, as they bring up an interface 
after shutdown or sleep or move) and interruption to connections will 
be minimised.
* One way is to leave the pool declarations in place and add "ignore 
booting" to them. Another is to simply remove the pool. If you remove 
the subnet declaration then clients will get a NACK, so they may 
switch address mid session and dropped connections are more likely - 
and see below for DNS implications.

With advanced planning, you can speed things up by reducing the lease 
time in advance - eg if you have 4 week leases, then reduce this in 
stages to perhaps only a day (or less) over the month or so prior to 
the changeover. That way, when you stop renewing leases for the old 
addresses, it won't take long for them to expire.

If you can't (or don't want to) do parallel running at all, then you 
will want to get clients onto really short leases. Reduce lease times 
in advance - eg gradually cut them down 
4wk-2wk-1wk-4day-2day-1day-12hr-3hr-... Ie, at t-4weeks, cut the 
lease time down to 2 weeks, at t-2weeks, cut the lease time down to 
1week and so on. Just remember that the shorter you get the lease 
times, the quicker your customers will notice if your server breaks !

At the appointed time, change configs - router interface addresses 
and DHCP server. Clients will lose connectivity until they lease a 
new address, but if your lease times are short, then this should be 
too much of a problem if you time it right AND inform customers 
accordingly. In a corporate environment I'd plan to do this when the 
lease number of people are on the systems - which will obviously 
depend on your shift patterns and countries served.

There will be problems with dynamic DNS entries - if you just delete 
subnet definitions then I've an idea that the server will delete the 
lease records, but might not delete the corresponding DNS entries. If 
you do a graceful changeover, then DNS entries will get deleted as 
leases expire, and new entries can then be created on the next client 

Sadly, the last time I had to do this, I was still using the broken 
SCO DHCP server - only does a NACK if it's the first query since 
server startup ! Had to walk round the office doing manual 
release/renew on all the PCs :-(

Simon Hobson

Visit for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

More information about the dhcp-users mailing list