shared-network question

Simon Hobson dhcp1 at
Mon Oct 23 16:33:03 UTC 2006

Eric Helm wrote:

>Maybe I misunderstand the shared-network, but its my understanding that
>with a shared-network, the DHCP server will hand out an IP from ANY
>subnet in the shared-network, so if the giaddr is from 1 of the subnets
>in the shared-network, shouldn't the server assign an address from
>either of the subnets?


>If so, couldn't I just not include a range
>statement for the subnet I don't want to assign addresses from?

Yes, but BOTH servers will respond, each with a different address. 
The client will pick one of the offers (probably the first one it 
receives) and go with it.

>Otherwise, if the shared-network doesn't work how I think it does, then
>I agree with you, there needs to be some way to determine what subnets
>to use since the giaddr will only be the primary address of the interface.

You need a mechanism so that one of the servers will offer an address 
to the IPTV box and ignore the other client. For the other server, it 
needs to ignore the iptv box and service the other client.

However you do it, if you need a client to be in a specific subnet of 
a shared network then you need a mechanism to tell the dhcp server(s) 
how to decide which client belongs in which subnet.

The thing to remember is that when the client starts out, it has no 
IP address and broadcasts to the broadcast address - 
it does not use (for example) a subnet broadcast 
address which is what I think you were expecting to be able to use.

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