Separates VLANs with the same IP-Range

perl-list perl-list at
Mon Apr 29 13:43:59 UTC 2019

That is not always 100% the case.  I've seen relay agents in ISP environments that never tell the client the location of the actual DHCP server.  They masquerade as the DHCP server swapping their own address for the GIADDR in the response packet.  Probably not proper RFC wise, but I have seen it.

----- Original Message -----
> From: "glenn satchell" <glenn.satchell at>
> To: "Users of ISC DHCP" <dhcp-users at>
> Sent: Monday, April 29, 2019 2:14:02 AM
> Subject: Re: Separates VLANs with the same IP-Range

> Remember the DHCP relay only handles relaying broadcast traffic from the
> client. This is the DHCPDISCOVER and initial response packets. Once the
> client gets an IP address it talks directly to the DHCP server for
> checking, renewing and acknowledging, the relay is no longer involved.

> regards,
> -glenn

> On 2019-04-28 02:02, Simon Hobson wrote:
> > german181 at wrote:

> >> So i remove the nat konstrukt and add a subnet for the second relay.

> > It's not entirely clear what your network topology is here. Are you
> > saying that there is NAT between the DHCP server and the clients ?

> > There must be no NAT between clients and server - the server needs to
> > be able to send unicast packets to the client and this cannot be done
> > if there is NAT in the path.
> > Also, you cannot have any overlap in IP addressing between networks -
> > normal packet routing doesn't handle this, and the ISC DHCP server
> > certainly doesn't.

> > Remember two two fundamental rules of IP are "addresses must be
> > globally unique"(1) and "any node should be able to address a packet
> > to any other node"(2). NAT breaks both of these.

> > 1 - In the context here, in the collection of networks your DHCP
> > server is to handle, all addresses must be unique.

> > 2 - Obviously these days subject to administrative restrictions (ie
> > firewall filters). But again for your context, the DHCP server and any
> > client it is to support must be able to address unicast packets to
> > each other and have them delivered.
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> >
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