Hi,

I would like to give my Linux Gateway/Firewall a static public IP provided by my ISP but I have a router as such:

<ISP>---<ADSL ROUTER>---<LINUX GATEWAY>---<LAN>

My ADSL router is an Alcatel Thomson Speedtouch and connects to my ISP via pppoa and automatically adds the static public IP to itself. So here's what I do to get the public IP address onto the Linux Gateway:

1) I modify the ADSL Router's IPs: I delete the pppoa-assigned public IP (WAN.WAN.WAN.WAN) and add a ficticious IP: WAN.WAN.WAN.2 (ends with number 2 but could also be 1 or 3, etc..)

2) I configure eth0 (connected to the ADSL router) on the Linux box as WAN.WAN.WAN.WAN (static public IP provided by my ISP) and configure eth1 (connected to local LAN) as 192.168.1.1 (for example).

3) the lan is a 192.168.1.0 subnet

This configuration works but I have doubts regarding it's stability and wonder if there are alternatives. Of course, this configuration fails if the ADSL router is rebooted because the pppoa reassigns the public IP to the router, thus conflicting with eth0's public IP.

Is there another way to set the Public IP on the Linux Gateway's eth0?

Is specifying "default server" to 192.168.1.1 in the ADSL router's NAT settings and leaving the public IP on the ADSL router feasible? In such a case, how should eth0/eth1 be configured?

I think my ADSL router also supports bridging. Could I use it in my case? How does it work and how should the configuration be?

Thanks in advance

Of what I understand of what you are trying to do, bridging is the way to go. That way you 'shut down' the router part of the modem and just use the modem capabilities.
You should be able to set brigde networking using to config software you received with your modem. If not see the vendors website or contact your isp.
I use a speedtouch 510 in the Netherlands and used this link to setup bridging:

Speedtouch support

There are some issues regarding settings if the modem regarding different isp in the Netherlands. So if you're not Dutch and not using BBnet/Tiscali network you should look for a config util on the international support pages: www.speedtouch.com or contact your isp

usually you can open up the web gui for your router and change the file name at the end of the address bar to config and that will get you to the bridge mode page. Tier 2 of your ISP can tell you.