Is there any way to Migrate Users from One Network to Another?

Simon Hobson dhcp1 at
Mon Aug 16 19:23:07 UTC 2010

Jason Frisvold wrote:

>On Aug 16, 2010, at 2:40 PM, Vineesh Viswanath Iyer wrote:
>>  that would not work with ip helper-address , if you have same 
>>scope and you are migrating the dhcp server , then , this method 
>>will work . But here you are trying to change the subnet , just by 
>>a different helper , you would never be able to achieve your goal ..
>How so?  The IP assigned to the end-user is based on the source IP 
>that the DHCP request comes in on.   That address is set via 

But you would have to define the two subnets as a shared-network - if 
you don't then you will find your DHCP is badly broken ! Once you 
define them as a shared network, then the server will accept the 
primary IP address (actually any address in either subnet) as 
identifying the client as being in that shared-network - and hence 
eligible to have an IP from any pool in either subnet.

One technique would be to just remove the pool in the original 
subnet. The server will not issue any new leases, and it will not 
NACK any requests for those IPs. However, I think it will also not 
ACK any either, and will not extend any existing leases.

This would allow clients with an existing lease to continue using 
those leases until they expire, but the clients would still be forced 
to change at the end of the lease. If that end time comes while the 
user is in the middle of several sessions with servers etc, then the 
user is going to be a tad annoyed when everything breaks !

By some very careful manipulations of lease times, it may be possible 
to arrange that existing leases expire at quiet times (eg overnight 
in an office that only has a day shift) - which would minimise 

There is one further option - but the OP probably doesn't have any 
control over it. The DHCP server, as a result of the internal 
achitecture, will allocate unused leased first, and 'top down' 
address wise. If the new subnet is numerically high than the old, 
then new clients will automatically get addresses in that subnet. Any 
client that has a lease (whether current or expired) will continue to 
get their old address.

If this process works for the situation in question, then it would 
probably be worth going through the leases file periodically and 
deleting all expired leases - that way the client will get a new 
address in the higher subnet should it return to the network.

Simon Hobson

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