shared-network question

Glenn Satchell Glenn.Satchell at
Mon Oct 23 14:50:25 UTC 2006

>Date: Mon, 23 Oct 2006 09:23:47 -0500
>From: Eric Helm <helmwork at>
>To: dhcp-users at
>Subject: shared-network question
>I'm working with a situation where there are 2 physically separate
>networks each with their own DHCP server. One is for IPTV Set-top boxes
>and the other is for customer Internet use.
>Due to some subscriber equipment limitations at one location, we are
>needing to merge these two networks on the same physical wire.
>So, the customer facing interface has multiple IP addresses assigned. I
>can specify multiple 'ip-helper' addresses and I would like to keep the
>DHCP servers separate still.
>So, my question is, can I use a shared-network statement on each server
>containing both subnets, but only allocate addresses for the Internet
>network from 1 server and the Set-top boxes from the other server by
>simply not putting a range statement in the subnet that I don't want to
>serve addresses for on each server?

Almost. You need a way to determine which devices will belong on each

When the device boots initially it sends a DHCPDISCOVER to the all-ones
broadcast address ( So there is no way to tell from
this which network the device belongs to. You can use class statements
with an allow and deny in the respective ranges or you could use host
statements to create static assignments. Yo may be able to use a class
assignmernt based on the first few octets of the mac address if you
know what type of hardware is in use.

Make sure that each server is authoritative for its subnet and not
authoritative for the other subnet otherwise it will NAK the requests.

You could easily combiune this onto one dhcp server. Create the two
subnet definitions on the one box. Any subnet specific things like dns
servers, etc, can be defined in the subnet scope.

Glenn Satchell     mailto:glenn.satchell at | Some days we are
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