network configuration issue - iptables

I have set up a network for our house using a gateway server with etch and two NICs, eth1 for the internal network and eth2 for the DSL modem.

I set up iptables with firewall-builder and all seems OK, but I can only ever access the web interface on the DSL modem from the gateway server directly after downing the internal network on eth1.

The modem's web interface is on 192.168.1.1 on eth2 and if I don't down eth1, the browser won't find it.

This cuts the internet connection for the rest of the network though which means I have to tell everyone in the house, and secondly, I have to go to the actual machine which is not in my study where my workstation is.

So my goal is to be able to connect to the modem on 192.168.1.1 from anywhere internally. How can I set up iptables to route packets from eth1 to eth2 just for 192.168.1.1 ? It's not obvious in firewall-builder!

Thanks
Adam

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