Migrating ip addresses over

Unanswered Question
May 4th, 2007

Hi all

I am in process of migrating ip addresses over to a new scheme, the pc's are currently in vlan 1 and so is the dhcp server. I am putting them in a new vlan and want the dhcp server to issue the vlan a new ip address scope, they will both need to be running until all clients in vlan 1 have been swapped over. How would I do this ,as I dont want the dhcp server to issue the new scope to vlan 1, if I use the ip helper command on the switch for the new vlan, will the dhcp server issue the addresses accordingly, or will i need to put in a second network card and connect it to the new vlan?

thanks for your help ?


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Overall Rating: 4.8 (4 ratings)
leighharrison Fri, 05/04/2007 - 02:32

Hi there Carl,

All you need to do is create the new scope on the dhcp server and put an IP helper-address of the server on to the switch.

When the dhcp packet is relayed, the switch will put in a message telling the dhcp server which network the request came from. The dhcp server will then dish out an address accordingly.

Let me know how it goes!


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carl_townshend Fri, 05/04/2007 - 04:46

So Can I have 2 scopes for both vlan 1 and 2 ,will i have to use ip helper address on both vlans so they dont all go to one vlan ?

mohammedmahmoud Fri, 05/04/2007 - 04:53


IP-helper won't fit for VLAN1 as the DHCP server is in the same VLAN and will get the broadcast request, i am not sure if this solution would work why won't you use 2 NICs for the 2 VLANs with 2 DHCP scopes.

HTH, please rate if it does help,

Mohammed Mahmoud.

leighharrison Fri, 05/04/2007 - 08:12

Hi there Carl,

You need to do this:

say the dhcp server is for example

on the switch:-

interface vlan 1

ip add (or whatever)

interface vlan 2

ip add (or whatever)

ip helper-address

When a dhcp request arrives on vlan 2, the switch will take the packet and forward it to the dhcp server. In the packet that is forwarded, the switch will tell the dhcp server which network it came from (in the example and the dchp will send back an address in that scope.

You need to do this only in the vlans that the dchp server is not on. This is because a dhcp request as a broadcast, and it will not pass between networks.

Hope this clarifies,


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carl_townshend Wed, 05/09/2007 - 06:39

If I do this though, when I add the 2 scopes to the dhcp server, will it definetly not pass the scope to vlan 1 ?

leighharrison Wed, 05/09/2007 - 06:54


I promise you it won't!! In the DHCP packet, there is a section that tells the server which network the request came from. This is written in the DCHP and is not read from layer 3 headers.

Trust me!!



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