Creating a class based on the relay IP

Simon Hobson dhcp1 at
Wed Jun 29 07:05:41 UTC 2011

Sven Seelemann wrote:

>I'm trying to set up a set of separate network segments that are 
>hosted from the same DHCP server within a shared network segment. 
>The DHCP discover messages come with the relay address embedded in 
>them (location 0x42 in the packet I believe).
>Is there a way to specify a class based on that?

Let me get this clear. You have a DHCP server on one physical 
network, and you want it to serve clients located on another physical 
network connected via one (or more) router ?

The setup for this is ... just define each subnet and the rest 
happens automagically. When the server received a relayed request, it 
can tell from the GIAddr (Gateway Interface Address) field which 
network (note I said network, not subnet) the client is connected to 
and can then select an address which is appropriate for whatever 
subnet(s) is defined there.

Now you mention "shared network segment", do you *REALLY* mean shared 
network ? To be clear, this means two or more IP subnets on the SAME 
BIT OF WIRE (and if you use them, same VLAN). Two subnets on 
different networks connected by a router is **NOT** a shared network.

If you do have a shared network then you CANNOT use GIAddr to 
determine which subnet on that network a client should be put in. 
Typically all requests will arrive with the same GIAddr - which is 
normally the primary IP address of the interface on the remote 

Simon Hobson

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