Connecting a socket with static IP on a WiFi network while mobile data is turned on

Question

I am required to design an application in Android which requires the phone to connect to a server by opening a socket. I am able to achieve this when I am just connected to the particular wifi network (ie the Wifi network which hosts the server ) but in a situation when I am connected to the wifi network and the Mobile data network I get a socket exception thrown as android tries to connect the socket over the mobile network

I have already been able to connect the device when its just connected to the wifi of the device that needs the socket connection to be established

  static class StartTCPconnection extends AsyncTask<Void, Void, Void> {
    final WeakReference<RemoteActivity> activity;

    StartTCPconnection(WeakReference<RemoteActivity> activity) {
        this.activity = activity;
    }


    @Override
    protected Void doInBackground(Void... voids) {
        try {
            socket = new Socket("192.168.4.1", 900);
            Log.d(TAG, "is socket connected ? ...." + socket.isConnected());
            printWriter = new PrintWriter(socket.getOutputStream(), true);
            Log.i(TAG, "Checking if socket is really connected " + (socket.getLocalSocketAddress()));
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
        if (socket != null) {
            if (socket.isConnected() && isWifi) {
                Log.d(TAG, "onPostExecute: " + socket.isConnected());
                Toast.makeText(activity.get(), "Connection established", Toast.LENGTH_SHORT).show();
                Log.e(TAG, "onPostExecute: " + activity.get().getSharedPreferences(Constants.REMOTE_SWITCH_SHARED_PREFERENCE, Context.MODE_PRIVATE).getInt(Constants.REMOTE_SWITCH_KEY, 99));
                if (activity.get().getSharedPreferences(Constants.REMOTE_SWITCH_SHARED_PREFERENCE, Context.MODE_PRIVATE).getInt(Constants.REMOTE_SWITCH_KEY, 1) == 1) {
                    activity.get().joyStickFragment.checkSocketInstance(socket);
                    activity.get().joyStickFragment.changeUIForConnect();
                } else if (activity.get().getSharedPreferences(Constants.REMOTE_SWITCH_SHARED_PREFERENCE, Context.MODE_PRIVATE).getInt(Constants.REMOTE_SWITCH_KEY, 1) == 2) {
                    Log.e(TAG, "onPostExecute:Check  " + socket.isConnected());
                    activity.get().buttonRemoteFragment.checkSocketInstance(socket);
                    activity.get().buttonRemoteFragment.changeUIForConnect();
                }
                activity.get().connectionIndicatorImage.setImageResource(R.drawable.avishkaar_logo_on);
                activity.get().wifiIndicator.setImageResource(R.drawable.wifi_connected_icon);
            }

        } else {
            //   Toast.makeText(activity.get(), "Wrong Wifi Network connected", Toast.LENGTH_SHORT).show();
        }

    }
}

The above mentioned code connects me to the socket if the only network available is the WiFi of the device and the mobile network is turned off

Steffen Ullrich Steffen Ullrich · Accepted Answer · 2019-09-29T07:47:44

There is nothing special from the programming perspective between connecting to a server which is in the local network and a server which is not. The only requirement is that the server is actually reachable in the first place, i.e. not in a private unreachable network and not blocked by a firewall or similar. And of course that the public reachable address of the server is used as destination in the program.

        socket = new Socket("192.168.4.1", 900);

192.168.4.1 is an address in a private network. This means it is not accessible from the internet, which also means that it cannot be reached if your are connecting with mobile data or if you use wifi within a different network (like a public hotspot).

To make a connection from outside this private network possible the server must be reachable from outside this network, i.e. needs to have a public routable IP address. If the server is in some typical home network this can be achieved with port forwarding in the router. For larger setups such servers are located at data centers directly reachable from the internet or (as a special case of this) in the cloud.

Connecting a socket with static IP on a WiFi network while mobile data is turned on

1 Answers