Re: DHCP server assigning addresses on remote subnet

The DHCP client broadcasts its request on the wire. If the DHCP server is on that segment it replies with an offer of an IP in the subnet of the interface on which the request was received.

As Anthony pointed out, if the DHCP server is not on the same subnet you need some device to forward the request to the DHCP server. This forwarded request will still have the IP address on which the broadcast was received, so the DHCP server will know which scope to use.

If the router has the "wrong" IP as you speculate, it doesn't really matter. The client needs an IP in the same IP subnet as that router if it is going to communicate beyond its local segment. If there is in fact no scope matching that IP, then of course the request will fail.

"Anthony [MVP]" <anthony@xxxxxxxxxxxx> wrote in message news:u4wQIjhtIHA.2292@xxxxxxxxxxxxxxxxxxxxxxx
Yes, based on the router interface it came from,
Anthony,
http://www.airdesk.co.uk


"Scott" <Scott@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message news:9F1E2F8F-B35F-45DB-8935-9C4276500001@xxxxxxxxxxxxxxxx
Anthony,

Sorry, I misstated that. I was just reiterating that the address is assigned
based on the scope that matches the IP subnet of the LAN IP Address for the
FE interface on the router? Just trying to figure out how it knows which
scope to associate with - in the example of many remote subnets and many
address scopes.

"Anthony [MVP]" wrote:

The IP Helper is the address of the DHCP server, so if that were set
incorrectly there would be no reply,
Anthony,
http://www.airdesk.co.uk


"Scott" <Scott@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:F0C16D18-E551-442D-87D3-15700E7A7819@xxxxxxxxxxxxxxxx
> So, based on that information. If there were many scopes created, and > the
> LAN
> IP Address was configured on the router was accidently set incorrect, > it
> would request addresses from the wrong scope? (based on the subnet of > the
> local LAN)?
>
> "Anthony [MVP]" wrote:
>
>> Scott,
>> Yes, it knows where the request came from so it can allocate an >> address
>> from
>> the correct range. The IP Helper is just forwarding the request, >> which
>> would
>> not cross the subnets otherwise,
>> Anthony,
>> http://www.airdesk.co.uk
>>
>>
>> "Scott" <Scott@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
>> news:17D37C0E-FED2-4DD3-9DCC-811A8E6F9DFE@xxxxxxxxxxxxxxxx
>> > Stupid Question:
>> >
>> > I have a remote subnet connected via a WAN link. The DHCP server is
>> > installed on the opposite side of the WAN link. The Cisco router is
>> > configured with the IP Helper Address command, to point to the DHCP
>> > server's
>> > IP Address, but does not specify an address range/subnet. The DHCP
>> > server
>> > is
>> > configured for multiple subnets. If the Cisco router is configured >> > only
>> > with
>> > the address of the DHCP server, and not any details about which >> > range
>> > to
>> > obtain addresses from, how does the DHCP know from which pool to >> > assign
>> > addresses to this remote subnet? Does it use the LAN address from >> > the
>> > router
>> > as an index?
>>
>>
>>






.

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