Java - Socket has 'connection timed out error' or 'no route to host' error with certain internet

I'm making a client/server pair with sockets to send and receive data back and forth. When I'm at home on my internet using two separate machines for client/server, it works fine as expected. Data is transmitted and so forth.

However, today when I was working at a local coffee shop (Second Cup, in case that's relevant), this did not work. I kept getting the following errors: either connection timed out, or no route to host.

Here is the relevant code:

Server:

public class TestServer {

public static void main(String[] args) throws Exception {

    TestServer myServer = new TestServer(); 
    myServer.run(); 

}

private void run() throws Exception {

    ServerSocket开发者_StackOverflow mySS = new ServerSocket(9999);     

    while(true) {

        Socket SS_accept = mySS.accept(); 

        BufferedReader myBR = new BufferedReader(new InputStreamReader(SS_accept.getInputStream()));

        String temp = myBR.readLine(); 
        System.out.println(temp); 

        if (temp!=null) {

            PrintStream serverPS = new PrintStream(SS_accept.getOutputStream()); 
            serverPS.println("Response received"); 

        }

    }
}

}

Client: (the relevant part)

//sends a command to the server, and returns the server's response
private String tellServer(String text) throws Exception {

    mySocket = new Socket("192.168.0.XXX", 9999); //the IPv4 address      

    //use the socket's outputStream to tell stuff to the server 
    PrintStream myPS = new PrintStream(mySocket.getOutputStream()); 
    myPS.println(text); 

    //the following code will get data back from the server
    BufferedReader clientBR = new BufferedReader(new InputStreamReader(mySocket.getInputStream())); 
    String temp = clientBR.readLine(); 

    if (temp!=null) {
        return temp; 
    } else {
        return ""; 
    }

}

It's pretty much as simple as can be. Again, as mentioned, at home on my internet it works fine - just do ipconfig, grab the IPv4 address, and put it in for the client. In coffee shops with free wifi, it doesn't work. I even fiddled with many different ports just in case.

Thanks for any help, I'm new to sockets and finding this confusing.

192.168.x.y is a local address. source

You need your home machines ip address as the INTERNET sees it.

When you're home next, go to http://www.whatismyip.com/ and see what it thinks you are.

Note that you might need to go onto your router and route traffic from your router to your machine for port 9999, since that's what you'll probably be hitting.

When you're running both the server and client on the same machine, you could use the loopback address, 127.0.0.1, when out at the coffee shop.

Using the loopback address and running the server and client on the one machine should work all the time, whether at home or out.

You could check your IP address as suggested by Total, but that will only stay the same if you have a static IP. If you aren't sure if you have a static or dynamic IP address, you probably have a dynamic IP address but you should check your IP address a few times over a week or so to observe any change.

Another alternative is to consider a free dns server e.g. http://freedns.afraid.org/ , set a job to update your IP address regularly with that service and use whatever domain name you have chosen to access your local server.

With either method of accessing your home network remotely, you'll need to forward traffic on 9999 to the relevant machine on your home network.

HTH :)