DHCP server on a different subnet

Simon Hobson dhcp1 at thehobsons.co.uk
Thu May 12 07:05:44 UTC 2011

Lyle Giese wrote:

>But if an interface is on the same broadcast domain, but on a 
>different subnet, how will that interface determine what subnet the 
>broadcast packet came from?  I don't think it can.

You would need a lot of shared network statements. Eg, if 
was the client's subnet, you might have :

shared-subnet ninety-six {
   subnet ... {
     option routers ;
     range ;
   subnet ... {
The DHCP server could be on any address in the subnet. 
It would recognise that the broadcast request arrived on that 
interface, and work out from the shared network that 
addresses are also valid on that wire. When broadcasting responses 
back to the client, it would use that interface.

When the client came to unicast a renewal, it would do so to the 
servers public IP (you may have to explicitly set server-identifier), 
and the packet would go via the normal routing to get to the server. 
When replying, the server would also send the packet back using it's 
normal routing table - it wouldn't have a direct attach route to a address since it doesn't have an IP in that range on the 
directly attached interface.

But I'm not entirely sure what this gets anyone, what sort of routing 
device doesn't have a BOOTP/DHCP Relay Agent ?

Simon Hobson

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