Pool selection based on Giaddr of different subnet

Simon Hobson dhcp1 at thehobsons.co.uk
Tue Mar 17 17:34:32 UTC 2009

Dario Aguilar wrote:

>But here it´s saying another thing...
>    Note that, in some network architectures (e.g., internets with more
>    than one IP subnet assigned to a physical network segment), it may be
>    the case that the DHCP client should be assigned an address from a
>    different subnet than the address recorded in 'giaddr'.  Thus, DHCP
>    does not require that the client be assigned as address from the
>    subnet in 'giaddr'.  A server is free to choose some other subnet,
>    and it is beyond the scope of the DHCP specification to describe ways
>    in which the assigned IP address might be chosen.

It's describing a shared network - the address 
used by the gateway is still the address of an 
interface on the same physical network as the 
client, and the dhcp server must be configured 
with a shared network declaration.

Eg, suppose the relay agent is a Linux box, and 
eth0 is the interface the client is connected to. 
Lets also assume it's a shared network, so you 
might see :


The GI Addr used by the relay agent can be any of 
:,, or

The DHCP server MUST be configured like this :

shared-network big-net {
   subnet netmask {
   subnet netmask {
   subnet netmask {

As far as teh DHCP server is concerned, all ip 
addresses in these subnets are equivalent - ie if 
you define a range in two or three subnets, then 
the server will NOT assign IPs from a specific 
range to a specific client without some mechanism 
(such as classes) to do it. So in that case, you 
CAN use a class to assign IPs from a specific 
range, but the IP must still be valid for a 
subnet in the shared network determined from the 
GI Addr.

Simon Hobson

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