DCHP host entry with 2 IP's ?

Chris Buxton chris.p.buxton at gmail.com
Fri Aug 5 22:20:13 UTC 2011

On Aug 5, 2011, at 1:41 PM, Paul Reilly wrote:
> > I'm not clear on how the DHCP server knows which subnet to use though?  Does it use the the router address of the subnet (where ip helper address is configured)  to determine which subnet the request is coming from?
> Yes, for remote subnets. For local subnets, it uses its own network address for the comparison.
> Yes, that makes sense for remote subnets. But for local subnets does it compare it to it's own IP address? 
> What if there's more than 1 local subnet?

Yes, that's what I meant when I said that it uses its own network address for the comparison.

If the DHCP server has more than one network interface, and thus more than one local subnet, each one is treated as a separate subnet.

> One other question which is bugging me, is if the lease time is set high, say 1 day,  and a laptop user in subnet 10  walks to another building and plugs in to subnet 20,  their machine still has the 1 day lease from subnet 10. From what I've seen in the DHCP logs, when the laptop starts up again, it doesn't do a DHCPDISCOVER but a DHCPREQUEST for it's subnet 10 address.  At the moment the DHCP sends back a DCHPACK saying yeah, that IP is fine, but it's clearly not, as it won't work in that subnet. What is the expected behaviour?

The expected behavior is that it should send a DHCPNAK, or "not acknowledged". Basically, it should reject the renewal, at which point the client should immediately start over with a DISCOVER.

> Should it reconfigure itself auto-magically for subnet 20, or does it's lease have to expire, or does the user  have to do a ipconfig /release then ipconfig /nenew  to force it to get it's new subnet 20 IP address?

You can force the previous lease to expire with the statement 'one-lease-per-client true;'. Either way, though, the client should automatically figure out that it's on a new subnet and get its other assigned address.

Chris Buxton
BlueCat Networks
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