Forward a port with modem (with built-in but disabled router) + a separate router

Question

I have a Motorola SBG6580 that is a modem and a wireless router in one. However, the wireless router part was bad so I disabled it and got a separate wireless router. I can go into the configuration pages of both the modem and the router now, and I'm confused as to which device needs to be configured for port-forwarding. I have a raspberry pi that I want to set up as a webserver.

Do I configure the router, the modem, or both?

Right now, the SBG6580's 1st LAN is connected to the wireless router's WAN, and the internet is working well. Note that the SBG6580 only has 4 ports, and I'm assuming they're LAN,as they are not labeled.

Answer 1

it depends... If the router is connected to the modem via the 'internet' or 'wan' port on the router, you would need to change the modem to 'bridge' mode, which hands off the public IP to your router. Then in the router configuration you would set up port forwarding.

If the router is connected to the modem via one of the router's LAN ports (no NATting going on) then you would set up the modem to do port forwarding.

Personally I recommend that the modem be bridged, and let the router do all the work, including PPPoE authentication if you are on AT&T DSL. This is a simpler config.

Search for instructions or call your ISP to determine how to put your modem in bridge mode. NOTE: bridged modems are no longer accessible via web interface. The modem just becomes an invisible 'forwarder' sending all packets to the router.

• laketuna: I've updated my post. My modem doesn't have distinguishing WAN vs LAN ports..
• Ryan Griggs: But your router probably does have WAN vs LAN ports. Your choice makes a huge difference.
• laketuna: the SBG modem doesn't label or show different placement of the ports on the back. However, I was able to get a hint from the term "bridge mode," which led me to some articles that described how to do it. Thanks!

Answer 2

Your SBG6580 needs configuring for internet access.

Assuming that you've plugged one LAN port from your SBG6580 into one LAN port on your WiFi device, then you're good to go.

If you have a WiFi device that also has a WAN port, and you've plugged the LAN of your SBG6580 into the WAN of your WiFi, then that's a different story (and you shouldn't do this unless you have a good reason).

• laketuna: I'm currently doing what you describe in the latter. e.g. The wireless router's WAN is plugged into the 1st LAN (I'm assuming it's LAN because it has 4 ports and they are not labeled or separated) on the SBG6580. I've been using it like this for a while. Is this wrong?
• Mark Henderson: @Youn you have just set up a double-nat scenario. You will need to do port forwarding twice then. Once from SBG to WiFi's IP address, then from the WiFi to your RPI. Yes this is bad, because it's two layers of abstraction when you only need one, and you've disjointed your network in such a way that you've now got this problem. Another solution is to disable NAT on your SBG6580 and pass through an IP address directly to your WiFi router. This depends on your ISP though, as they may only permit one MAC address per connection.
• laketuna: Oh OK, I see what I had set up now (double-nat)! I have disabled NAPT mode on my modem and reconnected my wireless router's WAN to the 1st LAN on my modem. I can no longer connect to the SBG6580 at 192.168.0.1 (as expected since its no longer a router?), but my wireless router seems to be working well.
• Mark Henderson: @Youn - your SBB is probably on a different subnet now and your WiFi router does not have a route to it, so it doesn't know how to get there. If this becomes a real issue, you will have to bypass your wifi and plug directly back into the SBB to get back into it. Or, worst case, they are both on the same subnet, in which case it is impossible to get there now because you can't have two interfaces on different networks on the same subnet and route between them.
• laketuna: SBB (did you mean SBG?) can no longer be accessed except by either a hard reset or direct connection (already forgot, but it was 2 hours ago). Anyhow, I think I have finally set things up with the new hardware, including a Raspberry Pi that was inactive for a long time.