How to ssh into a machine that is at different subnet?

Question

I'm connecting 2 different devices (laptop and desktop) to internet with my mobile phone. I connected my phone to my desktop via USB and enabled USB tethering. I've also enabled hotspot to connect my laptop via Wi-Fi. Now, both devices are connected to my phone and I want to ssh from one to other.

# my laptop
❯ ifconfig | grep 192 -B1
wlan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.43.186  netmask 255.255.255.0  broadcast 192.168.43.255

# my desktop
❯ ifconfig | grep 192 -B1
enp0s26u1u3: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.42.17  netmask 255.255.255.0  broadcast 192.168.42.255

In my laptop, I run:

ssh "[email protected]"

But it doesn't do anything. I've read that both devices needs to be in the same network in order to do this but I can't connect my desktop with wifi because I don't have a wifi adapter. With the current setup what should I change to make this work?

Answer 1

I've read that both devices needs to be in the same network in order to do this

That's an over-generalized statement. It's halfway-true in one sense, false in another.

"Same network" is different from "same subnet" – a single network can consist of many subnets, and if the routers are configured right then they'll allow devices from different subnets to communicate.

(Even then, "same network" is only important in the specific case of using private addressing such as 192.168.x, but not in general. For example, superuser.com is neither in your own subnet, nor even in your ISP's broader network, yet you're able to connect to it.)

However, a router might explicitly forbid some communication between the subnets that it manages, usually by using firewall rules which selectively accept or block specific kinds of traffic. (Often that's the whole point of dividing a network into subnets.) That's what Android does – it specifically blocks traffic between the "USB tethering" interface and the "Wi-Fi hotspot" one.

So the reason your devices cannot connect to each other isn't because they are on different subnets in general – but because your router (which is the Android phone) is configured to prevent communications between different subnets specifically.