DCHP host entry with 2 IP's ?

Paul Reilly pareilly at tcd.ie
Fri Aug 5 20:41:41 UTC 2011

> Is it actually not working? If so, try separating the fixed addresses into
> multiple declarations:
> Yeah def not working. It's handing out the 20 subnet address to a request
from the 10 subnet.
I'll try seperating out the host declarations in to two, like you say.

> host myhost1 {
>    hardware ethernet 00:1C:23:EE:C1:8E;
>    fixed-address x.y.10.10;
> }
> host myhost1 {
>    hardware ethernet 00:1C:23:EE:C1:8E;
>    fixed-address x.y.20.10;
> }
> 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?

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? 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

Thanks for all your help,

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