dhcp config - 2 subnets on one phys net

Simon Hobson dhcp1 at thehobsons.co.uk
Fri Sep 29 07:37:26 UTC 2006

Igor Antokhin wrote:

>Sorry for a question from a newbie, but I could not find a clear answer to
>mu problem - only pieces of the puzzle...
>I have a computer on a physical network It runs Linux
>Fedora Core 5. It is just a client machine not providing any general
>use services like dns printing etc. Now for some reasons I want to create
>a private network for my department using the same physical ethernet.
>I understand that what I have to do is this:

The problem is that you CANNOT provide DHCP to only one of the 
subnets - at least not without co-ordination with the admin of the 
existing DHCP.

The problem is that even though you can apparently run two interfaces 
(one 'real', one 'virtual'), DHCP relies on broadcasts which do not 
respect the distinction between the subnets/interfaces.

To make it work you will have to be able to identify every client 
that will be part of your private network (MAC address is usually 
easiest). On your new dhcp server you will have to service ONLY those 
clients and ignore all others. The admin of the existing dhcp server 
will have to explicitly ignore your clients. If you don't do this 
then you will have two dhcp servers fighting each other - yours will 
offer a private address, the other will offer a public address, and 
whichever the client decides to accept, one or other dhcp server will 
reply with a NAK.

Unless you specifically want to play with the dhcp, you might be 
better talking with the network admin and see if he can just set up 
the dhcp for your private network on the existing server - be a whole 
lot less effort overall.

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