DHCP and 2 subnets

Chris Arnold carnold at electrichendrix.com
Sun Apr 6 18:37:06 UTC 2008

On Sunday 06 April 2008 08:34:03 am Simon Hobson wrote:
> On the server, you simply define both subnets<period>

OK , so does my below config look right:
option domain-name "domain-here";
option domain-name-servers 192.168.123.x;
option ntp-servers 192.168.123.x;
ddns-update-style none;
subnet netmask {
option routers 192.168.124.x;
default-lease-time 14400;
max-lease-time 172800;
subnet netmask {
option routers 192.168.123.x;
default-lease-time 14400;
max-lease-time 172800;
> The trick is that you need to configure your router (or actually ANY
> device on the 'remote' network (192.168.123.x)) as a DHCP Relay (or
> BOOTP Relay). Note that this is NOT the same as forwarding broadcasts
> which will NOT work.

I have configured our router for DHCP Relay Agent but a 192.168.123.x client 
still gets a 192.168.124.x address. I have the router config set to point to 
the gateway of the 192.168.123.x network. Here is the help on dhcp relay 
agent from the firewall:
The NetScreen Device as a DHCP Relay Agent

When acting as a Dynamic Host Configuration Protocol (DHCP) relay agent, the 
NetScreen device forwards DHCP requests and assignments between hosts in the 
Trust zone and a DHCP server in the Untrust zone (**all my action is between 
the trust zones**). The DHCP messages between the NetScreen device and the 
DHCP server can be transmitted in the open or through a VPN tunnel.

You can configure up to three DHCP servers for the DHCP relay agent. The relay 
agent unicasts an address request from a DHCP client to all configured DHCP 
servers. The relay agent forwards to the client the first response from a 

You can configure the DHCP relay agent on any physical or VLAN interface.

> Sounds like you have configured the router to forward broadcasts -
> this will NOT work as explained above.

From the reading of the router, it looks like it is forwarding broadcast which 
will not work?

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