Server on Android reachable from devices not on local network?

Question

I have been trying to set up an app that allows communication between two devices, a server and a client. I have established the connection and can send messages between the two as long as they are connected to the same source of internet. When I connect the devices to two different sources, I am not able to establish a connection between the two. It works by entering the IP address of the server you want to connect to.

For example, if my server device is connected to my home Wi-Fi and my client device is connected to the same Wi-Fi router, communication works as supposed to. But, when I connect the server to Wi-Fi and my client to cellular data (or different Wi-Fi router), the client is not able to find the server.

From all the searching I've done, I found that it is because my server is bound to my local host. So my question is, How can I make my server reachable from anywhere regardless of what network you are connected to?

I used this tutorial to get my server running:

http://android-er.blogspot.com/2014/02/android-sercerclient-example-server.html

The very first comment is a question that asks why it only works on a local network and someone answered saying you have to set up port forwarding on your router. From what I understand, this will work only if the server device is always connected to the same router. This will not be the case for my app. The server will have a new IP address as the device changes networks. I want this IP address to always be reachable no matter what network the server is connected to.

Answer 1

You will need to create a relay server. This would be set up using a java application on the PC and it would manage virtual "rooms" that then relay the messages to the clients. The computer you run your relay server off of will need port forwarding but the clients will not. Much of the code from that tutorial could be applied to this concept.

See: How to create a java Server that accepts client connections and then build a relay connection for a client pair

Answer 2

If your server is behind a firewall, you will not be able to reach it unless that port is open on the firewall. The only way to avoid this is to have a non-firewalled server.