Setting my router behind my isp

Hi, as the title says I want to put my router behind my ISP modem, they won't let me set bridge mode but I can set firewall rules, routing and other things. One important thing is that I can see my server from the outside, so this is a good change from my previous service.
The modem has some options, but it's not like openwrt, so I'm trying to make it as much of a bridge as I can.
From what I read from other post, I should connect my router to the bridge using the wan port, set a static ip. In the modem, I set my router to a DMZ.
Is there anything else I should do from the modem?

  • What problems can I expect with my server?
  • Is there a way to set the ddns script? How does the router see the external IP?
  • Will I lose a lot of performance? network speed? I don't normally play network games.
  • Any other advice?

You should ask your paid ISP to characterise their router in your measure system

Does your ISP device have the option for static routes? If so, you can disable masquerading on your OpenWrt router's wan firewall zone, thus avoiding double-NAT.

Hopefully none, but this assumes your ISP is giving you a proper public IP and that they're not blocking any ports. Obviously make sure that you've got your server properly secured for any services that will be exposed to the internet.

Yes, not a problem. The ddns scripts can use an external website to do the test. The page that renders will specifically echo back the IP address from which the request was made. This works as long as you have a true public IP (otherwise, you get the 'apparent public IP' if your ISP uses NAT/CG-NAT).

You shouldn't, no. It might marginally increase latency, but as long as your device isn't resource constrained, it should be truly minimal.

1 Like

I have this options

Yes, the static route (first screenshot) is what we want. That's good news.

Sorry the last image is from service route configuration

The rule should be like this?

if my openwrt router is on from the modem perspective?

What is the lan ip of the OpenWrt router? as seen from the modem

Please connect to your OpenWrt device using ssh and copy the output of the following commands and post it here using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have:

ubus call system board
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall

I'm really sorry, but the router isn't connected to the modem. I was going to set the static IP from the modem to be, but I'll do it tomorrow. I'm going to bed now, but I'm really, really thankful for your help.

Ok. What I’m trying to understand is the OpenWrt router’s lan address. Not the wan addrsss (which will be on the isp router’s lan)

ok, from my pc when I connect it to the router, the router will be, I will change the modem to be in the network
In the modems dhcp I can set the router ip to be static, I don't know if this is important or not.
This helps?

ps: I have my home network in the network. That why I have the modem with the now, but i will change it.

The OpenWrt router must have a different subnet on its lan than your ISP router's lan.

In other words, if the ISP router's lan is, your OpenWrt router must be a non-overlapping subnet such as its default network.

The OpenWrt router's wan will have an address on the lan of the ISP router. So this means that the OpenWrt router's wan will be something like as long as your ISP router's network stays as it is.

In this situation, the static route you will enter into your ISP's router will be: via

(or alternatively):

IP address:
Subnet Mask:
WAN Name: <probably something that indicates that this happens on the lan interface; need to see what else is in that dropdown>

These are the options in the dropdown menu

Tomorrow I will test it, thanks

should I do something in my openwrt wan interface?

probably br0

The interface, no. But once the static route is set, you will go to the wan firewall zone and turn off masquerading.

1 Like

Hi, how can I check if the routing is working or not?

When you turn off masquerading on the wan firewall zone (on the openwrt device), that will be your test. If the internet access still works from behind openwrt, the static route is working. If the internet stops working, then the route is not functioning as expected.