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I've been struggling with an issue here at my office for at least a couple of days, and I've tried googling to no avail. I'm admittedly more of a coder than a network guy, so please bear with me!

Basically, my issue is that I've currently got a BT Infinity router (made by 2wire) which runs PPPoE as is assigned an external IP address via DHCP each time it connects (in the 80.x.x.x range). However, BT have also allocated us a block of 5 external IP addresses (in the 217.x.x.x range) that we can use to route external traffic properly.

The BT router has an option to enter the IP Address and subnet mask of this block, and it automatically forwards traffic to the corresponding network adapter on the internal LAN.

For example: If I telnet to 217.1.1.1 port 25 (external ip), it passes through the router (and I assume some form of NAT?) to a network card on our mail server (Windows 2003) which has been configured to listen directly on 217.1.1.1.

Now, I'm wanting to replace our (possibly crappy) BT router with a good old Linksys WRT54GL box running Tomato, but I just can't seem to figure out how to replicate this functionality. I suspect I need to add a static route onto the tomato box, and I've tried copying the routes from the BT box to Tomato without any luck. I've looked at using 1:1 NAT, but I'm not happy with that solution as we run a few services that require the external IP to be configured directly on the box (VPN for example).

So far I've managed to SSH into Tomato, add one of the external IP addresses to the ppp0 adaptor and I was able to ping that externally, but I just can't seem to open ports or get the traffic to go beyond the router onto the LAN.

Any help will be greatly appreciated! :)

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You can either use 1:1 NAT or no NAT at all, meaning classical routing. In this case just assign your 217... subnet to the LAN interface and disable NAT. If your firewall setting allows it your router will start routing then :) –  dog Dec 13 '11 at 0:59
    
Ah, that does make sense! However I have a another set of machines on the 192.x range that need NAT to access the Internet through this router. I could just assign a 192.x address to the LAN interface as well, but I don't think this would route traffic as NAT would be disabled? –  Soen Trueman Dec 13 '11 at 8:22
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