We help IT Professionals succeed at work.

DHCP and IP Helper

I have sliced up my new subnet as follows

Datacenter - 10.10.1.x/24
Everything in the data center will be static from this subnet

Outside the data center - 10.10.2.x/23

This will include switches, printers and users

Now the DHCP server is obviously going to be sitting on 10.10.1.x/24 but will need to hand out 10.10.2.x/23 addresses.

On my layer 2/3 switch I have multiple interfaces on VLAN1 and I believe all I need to do is on the VLAN1 interface

ip helper-address (this is the IP of the DHCP server)

And that should be that yes?
Watch Question

Les MooreSr. Systems Engineer
Top Expert 2008

Assuming that your 10.10.1.x network is VLAN1
and the 10.10.2.x/23 network will be VLAN2

interface vlan 1
 ip address
interface vlan 2
 ip address
 ip helper-address

The server will have to have its default gateway point to this switch vlan interface IP

You got it almost right ;-)

It's the machines in 10.10.2.x/23 segment that need to find the DHCP server that's in the 10.10.1.x/24 segment. So you have to put the "ip helper-address" in the other VLAN.


There is only one VLAN - VLAN1 which houses secondary IP's for the other subnet
Then the switch wil automaticly route the traffic between the 2 segments. But DHCP requests won't be routed. You need the ip helper-address command for that.

And the ip helper-address works only if you put it in the segment that can't find the DHCP server now. And you can only put it in that segment if it has a VLAN.

So I think you need to create a VLAN for your 10.10.2.x/23 segment, so you can put the ip helper-address command in that segment to point the direction to the DHCP server.
Since you have 2 different subnets, you need to route packet between them.
On a layer 3 switch this is accomplished by setting several VLans and enabling inter VLan routing.

If you have a router between your 2 subnets, the ip helper setting must be set there.

Maybe check this page:


I am going to be performing this after this first of the year. I'll add VLAN2 and then assign the new IP and IP Helper address to that interface.