Using a DHCP server for two subnets

[Matt Fahrner]

Unfortunately if they share a single broadcast domain, that is, if one 
subnet can hear the other subnet's broadcasts (which seems to be what 
you're implying), then in my experience there is no way to hand out the 
DHCP addresses cleanly on a per subnet basis.

The problem is DHCP requests are layer 2 broadcasts and do not contain 
any IP network level information. Thus it's impossible to tell which of 
the two subnets the DHCP broadcast request came from. Therefor you 
couldn't know which pool to allocate from.

That's not to say you can't serve DHCP, you can, but the dynamic pool 
has to be applied to one of the two networks only (or rather, if not, 
you'll never be able to reliably predict which network pool a given 
client is going to be allocated from). If you want this form, look into 
the "shared-network" syntax.

You might get it to partially work doing one of the network with 
hardcoded MAC to host relationships (or perhaps client identifier, I 
can't remember all the options for keying). Still, there is no way to 
cleanly dynamically allocate on both networks with some regularity.

If you are using VLANs, unless you have some hosts mixed on a hub, why 
not just split them completely so they don't share broadcast domains? 
Extra NIC cards are cheap or you could look into using VLANs straight 
into your server, which Linux supports.

			- Matt

Elizabeth.Brosch at thomson.com wrote:
> Two subnets sharing the same wire and interface on the dhcp server.
> Well actually my dhcp server has one interface configured with a
> 10.225.248.x IP address.  I want to serve hosts on the 10.225.240.x
> subnet.
> 
> The 248 and the 240 subnets are on the same vlan.
> 
> Liz Brosch
> TSH System Services
> Philadelphia, PA  19104
> office:  (215) 823-6144
> cell:     (267) 784-9166
> -----Original Message-----
> 
> 
> _______________________________________________
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