If you use a router for DHCP request forwarding using BOOTP IP Helper
Address, the DHCP server will accurately assign IP addresses to the correct
subnets from the correct scopes using the source IP address.
That is unless the request comes from an Ethernet interface has multiple IP
addresses assigned from different logical subnets. In that case, it seems
to always assign an address from the scope of the lowest numerical IP
network address assigned to that interface.
Vern Stitt
AE, ASE, CCNA, MCSE
""Dale E. Drummond"" <[EMAIL PROTECTED]> wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> The DHCP server knows what scope to pick the ip address from by looking at
> the GIADDR field in the DHCP packet. When the router helpers a DHCP
> request to the DHCP server it populates the GIADDR address with the
> primary ip address of the router interface that the DHCP broadcast request
> was received on. When the DHCP server receives the request it looks at
> the GIADDR field then looks to see if it has a scope that matches the ip
> subnet of the address in the GIADDR fields. If it does it assigns an ip
> from that subnet.
>
> Hope this helps.
>
> Dale
>
>
>
> Date: Sun, 16 Jul 2000 20:35:20 -0500
> From: [EMAIL PROTECTED]
> To: [EMAIL PROTECTED]
> Cc: [EMAIL PROTECTED]
> Subject: RE: DHCP and subnets
>
> [ The following text is in the "iso-8859-1" character set. ]
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>
> My 2c on this .
>
> When the DHCP broadcasts crosses the router ,it carries that routers mask
> and ip info across to the dhcp server ( assuming helpder is there)
>
> the DHCP server on seeing that the reqest came from that particular subnet
> issues an ip from a scope ...U could configure multiple scopes and combine
> them into one superscope to do this. Hope this helps.
>
> Cheers,Padhu
>
> -----Original Message-----
> From: whatshakin
> To: [EMAIL PROTECTED]
> Sent: 7/16/00 12:11 PM
> Subject: DHCP and subnets
>
> Hello folks,
> Please clarify this for me.
>
> Hypothetical example: Campus LAN with multiple buildings. Each building
> on its own vlan and with its own subnet addressing scheme. All buildings
> tied in to a Catalyst 5500 which has RSP doing all the inter-vlan
> routing. Data center using a single DHCP server with multiple scopes
> (one scope per vlan/subnet etc) to supply all vlans/subnets with their
> respective ip addresses. I want to understand how the DHCP server knows
> how to hand out the correct ip address from the corresponding subnet to
> the workstations that request them. I have come to believe that
> initially DHCP servers have no idea whom is requesting an address, they
> just hand them out to whoever asks...this is what is confusing. I
> understand that each Vlan needs its own gateway address where the
> workstations aim their broadcasts and there an ip helper-address
> statement in the RSP for each vlan, but I still don't understand how the
> DHCP server knows how to hand out the appropriate address when it has
> multiple scopes enabled.
>
> TIA for any clarification you can offer.
>
>
> ------------------------------------------------------------------------
> Dale E Drummond [EMAIL PROTECTED]
> http://www.pobox.com/~dale/
> ------------------------------------------------------------------------
>
>
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