Re: DHCP serving more than one subnet (longish)

From: JF Mezei (jfmezei.spamnot_at_istop.com)
Date: 10/24/03 

Date: Fri, 24 Oct 2003 17:45:35 -0400

Mark Berryman wrote:
> DHCP packets are formatted the same as any other IP packet. The only
> real difference is that the source address is all zeros and the
> destination address is all ones (which means it is a broadcast packet).

Responses from the server however rely on the ethernet address only since
there is no IP address to send the response to. (responses are not
broadcasts). So while it may have the same format as an IP packet, it isn't
one per say when it comes to issues related to routers.

> Optional. The response can be either broadcast or addressed directly.

Nop. Responses are sent to the client's ethernet address or to the ip address
of a relay server if the giaddr field is not null. In the later case, the
relay server will issue an ethernet packet on the other ethernet segment that
is adressed to the client directly.

> I think you have a semantic issue here. The response packet is a
> properly formatted IP packet. If it weren't, no router would pass it
> and DHCP requests can certainly be relayed via a router.

Nop. DHCP requests cannot be passed by a ROUTER. You need a relay server
(which I assume can be integrated into a router, but it isn't part of the
basic role of a router).

> Not quite. The relay agent simply enters info into the gateway field.

I stand corrected. giaddr is filled with the relay server's IP address on the
other side of the client's lan. Interstsingly, the relay server RFC did
mention the addition of options in the client originated messages.

> This tells the DHCP server what subnet the requesting host is part of
> and also how to return the packet.

It doesn't directly tell it what subnet it is coming from. For instance,
consider a router that have 4 interfaces. The IP address in the giaddr would
be the IP address of the interface talking to the DHCP server and the DHCP
server would then not know which of the other 3 subnets the client request
originated from. (I think that this is where those relay server options come in)

> The bottom line: dynamic addressing and multiple subnets on the same LAN
> do not mix. Only one of the subnets can be dynamic, the other(s) must
> be static.

Not quite. You should be able to define a whole bunch of ethernet adresses as
belonging to a group, and that group has its own range of dynamic IP adresses.
However, on the VMS DHCP server, I haven't quite figured out how to associate
a group with an IP range.

Relevant Pages

  • Re: DHCP serving more than one subnet (longish)
    ... >>destination address is all ones (which means it is a broadcast packet). ... > Responses from the server however rely on the ethernet address only since ... DHCP works over media other than ethernet. ... the case if the request is relayed through a router]. ...
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