Simple Java Client/Server Program

Question

I'm writing my first java client/server program which just establishes a connection with the server sends it a sentence and the server sends the sentence back all capitalized. This is actually an example straight out of the book, and it works well and fine when I'm running the client and server on the same machine and using localhost for the server address. But when I put the client program on a different computer, it times out and never makes a connection with the server. I'm not sure why this is and its kind of lame making a your first client/server program and not actually be able to use it on two different machines. Here is the client code:

import java.io.*;
import java.net.*;

public class TCPClient {
    public static void main(String argv[]) throws Exception {
        String sentence;
        String modifiedSentence;
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));

        Socket clientSocket = new Socket("localhost", 6789);
        DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
        BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

        sentence = inFromUser.readLine();
        outToServer.writeBytes(sentence + '\n');
        modifiedSentence = inFromServer.readLine();
        System.out.println(modifiedSentence);
        clientSocket.close();
    }
}

Here is the server code:

import java.io.*;
import java.net.*;

public class TCPServer {
    public static void main(String args[]) throws Exception {
        String clientSentence;
        String capitalizedSentence;
        ServerSocket welcomeSocket = new ServerSocket(6789);

        while(true) {
            Socket connectionSocket = welcomeSocket.accept();
            BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
            DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
            clientSentence = inFromClient.readLine();
            capitalizedSentence = clientSentence.toUpperCase() + '\n';
            outToClient.writeBytes(capitalizedSentence);
        }
    }
}

The only thing I change when I run it on two different machines is the client program makes its socket with the IP address of the machine with the server program (which I got from whatismyipaddress.com). Thanks a lot for any help.

Update: I am indeed on a campus and it seems that its probably not allowing me to use that random port. Any suggestions on finding out what port I can use and or a port that is more than likely allowed?

Answer 1

It's probably a firewall issue. Make sure you port forward the port you want to connect to on the server side. localhost maps directly to an ip and also moves through your network stack. You're changing some text in your code but the way your program is working is fundamentally the same.

Answer 2

There is a fundamental concept of IP routing: You must have a unique IP address if you want your machine to be reachable via the Internet. This is called a "Public IP Address". "www.whatismyipaddress.com" will give you this. If your server is behind some default gateway, IP packets would reach you via that router. You can not be reached via your private IP address from the outside world. You should note that private IP addresses of client and server may be same as long as their corresponding default gateways have different addresses (that's why IPv4 is still in effect) I guess you're trying to ping from your private address of your client to the public IP address of the server (provided by whatismyipaddress.com). This is not feasible. In order to achieve this, a mapping from private to public address is required, a process called Network Address Translation or NAT in short. This is configured in Firewall or Router. You can create your own private network (say via wifi). In this case, since your client and server would be on the same logical network, no private to public address translation would be required and hence you can communicate using your private IP addresses only.

Answer 3

If you got your IP address from an external web site (http://whatismyipaddress.com/), you have your external IP address. If your server is on the same local network, you may need an internal IP address instead. Local IP addresses look like 10.X.X.X, 172.X.X.X, or 192.168.X.X.

Try the suggestions on this page to find what your machine thinks its IP address is.

Answer 4

Instead of using the IP address from whatismyipaddress.com, what if you just get the IP address directly from the machine and plug that in? whatismyipaddress.com will give you the address of your router (I'm assuming you're on a home network). I don't think port forwarding will work since your request will come from within the network, not outside.