Unable to connect to device which is on the same router in network using external IP and external port

Question

I am not sure if this is the right place to ask this question but here goes.

I have a modem with ONR fibre connected to the internet.

It has 4 Lan ports. 1, 2, 3, 4.

I have 1 PC and 3 routers connected to the modem.

1 PC, 2 Router (Asus TUF AX3000), 3 Router, 4 Router.

The 3 routers are connected in AP mode. 2 asus routers and 1 provided by my ISP.

I recently changed the modem given by my ISP when I recontracted with them. Previously they gave me a Huawei modem (Huawei HG8240T5 ONR), now I am using a Nokia modem (G-240G-E).

I have already gone through 2 huawei modems over the last 4 years and I have not had this issue I am about to describe. It only started happening today when I switched over to the new modem. The settings on the modem have been left at factory, I only added some port forwarding stuff. All the routers were also left the same. I was able to connect to the internet on all my devices at first and browse with no issue.

So let's say my public IP address is 220.221.222.223.

I have a laptop connected over wifi to the 2 Router. Let's say my internet port for the laptop is 192.168.1.11. I have a ssh server running on it on port 22. I also set up port forwarding on the modem from port 22 to port 6111 outside.

Now usually to connect to the ssh server locally, I usually do ssh [email protected], or [email protected] -p 6111. Both of these should work locally no matter which router I am connected to.

If I am on another network, only [email protected] -p 6111 should work.

So I tested it outside and at home.

If I am outside,[email protected] -p 6111 works, as it should.

If I am on another router at home, [email protected] -p 6111 or ssh [email protected] works.

However, If I am connected to the same router, 2 Router, for some reason only ssh [email protected] works, I can't seem to use [email protected] -p 6111 anymore.

I am also able to connect from my 1 PC which is at 192.168.1.10 to the wireless device connected to 2 Router using both [email protected] -p 6111 or ssh [email protected] works.

I am also able to connect to the "1 PC" from the router at "2 Router" using both [email protected] -p 6110 or ssh [email protected].

The issue seems to be with devices connect to 2 Router.

Does anyone know what the reason for this is? I have tried factory resetting all my routers and only putting them to AP mode but this issue still persists.

I googled around and I figured that there is a feature called NAT hairpinning, I'm not sure if this is it? What should I do?

Answer 1

You seem to be trying to do NAT hairpinning:

Hairpinning is where a machine on the LAN is able to access another machine on the LAN via the external IP address of the LAN/router (with port forwarding set up on the router to direct requests to the appropriate machine on the LAN).

Not all commercial routers support this option. Evidently the Nokia modem doesn't. If you can't find any setting for enabling hairpinning (or NAT loopback) in the login page of the Nokia modem, then have the modem replaced with one that does support it.