more than one ip range

Simon Hobson dhcp1 at
Wed Apr 27 07:02:01 UTC 2011

Usuário do Sistema wrote:
>hi everyone, I'm in an project of network segmentation in some VLANS.
>so...I need set my DHCP server distribute more than one IP range.
>for exemplo, currently my server DHCP is inside to VLAN what is VLAN 1
>( ) it's distributes only on this VLAN 1.
>I wish that it distributes other ip ranges for other VLANs but on same
>NIC ( Interface  - VLAN 1 )
>for exemplo, Users from VLAN 2 ( network ) will be ask
>IP on server DHCP interface that has IP
>in my switch structure all it's ready....I have switch Cisco and I've
>configuration ip helper address in my vlan 1.
>how can I reach this scenery....more than one ip range on my server DHCP ??

This is very basis DHCP stuff.

You need three things for this to work :
1) On the "remote" network you need a relay agent 
- this picks up the local broadcasts, fills in a 
field to identify the network to the DHCP server, 
and forwards it. It sounds like you already have 
this in your switch.

2) You configure each subnet in the server, like this :

subnet ...
subnet ...

That's all there is to it - it doesn't matter 
which subnets are locally attached and which are 
remote, the server will work that out.

3) You need appropriate router(s) and link(s) 
that will allow unicast traffic between the 
clients on the remote networks and the server - 
this is needed to allow the clients to renew 
leases which is done by unicast rather than 

With these in place you'll find that it all happens automagically.

In a bit more detail, the key is that in the DHCP 
discovery and request packets there is a field 
called GI Addr (Gateway Interface Address). In 
requests from clients on a local network this 
will be blank and the server detects the correct 
subnet according to the interface the packet was 
received on. For remote networks, the relay agent 
fills in it's local IP address on the remote 
network and the server uses that both to 
determine the right subnet, and to return the 
packet to the gateway so it can broadcast it to 
the client.

It is well worth getting hold of the book "The 
DHCP Handboook" by Ralph Droms and Ted Lemon. 
It's very well written and explains all the 
details clearly - including some historical stuff 
that explains why it's done the way it is.

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