Using Class to have ip address on unknown network segments

Niall O'Reilly niall.oreilly at
Fri Feb 20 10:13:00 UTC 2015

At Thu, 19 Feb 2015 16:39:39 -1000,
Teva A-T wrote:
> I'm trying to use class to provide ip address to LTE customers which
> is on a network segment unknown by the dhcpd server.

  Stop trying.  It can't work.

> Here below what i can see when my LTE customers is sending its DHCP
> Discover : 
> dhcpd: DHCPDISCOVER from 50:00:00:00:00:04 via unknown
> network segment
> So indeed, is not a network segment known by the dhcpd.
> is just a relay agent,

  It's rather more than "just a relay agent". This is the GIADDR field
  of the DHCP message, set by the relay agent to identify the network
  segment where the client is connected.

  In other words, the relayed request means, "please give an address
  for a client on THIS network."

  Part of the DHCP server configuration ('shared-network' and 'subnet'
  sections) models the network topology.  The server uses GIADDR to
  identify which part of this model is the one from which an address
  for the client must be assigned.

> and what i want is to provide to my
> LTE cust an ip in subnet For that, i thought that i
> could use class, and then allow members of this class to get an ip,
> even if the network segment is unknown.

  You can use class to select the subnet, but you need a different

  The GIADDR value presented by the relay must belong to a known
  subnet. This may be either the same subnet as is intended for
  clients, or another one on the same shared network.

  It seems you will need to add a subnet to the shared-network
  definition you alrady have, so:

shared-network lte-cust {

option domain-name-servers,;
default-lease-time 360;
max-lease-time 360;

subnet netmask {
option routers;
option subnet-mask;
option broadcast-address;

subnet netmask {
    # minimal (/32) subnet for relay
    # you may need a shorter prefix

pool {
allow members of "lte";


  I've just seen that Simon Hobson got to press 'Send' while I was
  still typing.  As usual, his advice is excellent. Since there are
  one or two differences in both detail emphasis between his advice
  and mine, I may as well send anyway ...

  Best regards,
  Niall O'Reilly

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