communicate 2 PCs with P2P protocol

One PC is a server, the other is a client, they communicate using the public IP IPv6.

The example is implemented in Java

Attention: The Java file must have the same name as the class and respect upper and lower case.

First the server must be listening on a port:

java code ServerP2P.java:

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

public class ServerP2P {
    public static void main(String[] args) {
        try {
            // Create a server socket on port 15000
            ServerSocket serverSocket = new ServerSocket(15000);
            System.out.println("Server started on port 15000");

            // Accept connections
            Socket socket = serverSocket.accept();
            System.out.println("Connection established");

           // Send and receive data
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            PrintWriter writer = new PrintWriter(socket.getOutputStream(), true);

            // Send message
            writer.println("Hello from server");

            // Receive message
            String mensaje = reader.readLine();
            System.out.println("Message received:" + mensaje);

            // Close sockets
            socket.close();
            serverSocket.close();
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

compile the file

 javac ServerP2P.java

then run the code:

java ServerP2P

In the following example “2801:812:572:793:743f:8544:f9d4:45d” is the public IPv6 IP, replace it with the public IPv6 IP you want, you can find it on the following pages:

https://www.ipify.org

https://www.whatismyip.com

java code of ClientP2P.java:

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

public class ClientP2P {
    public static void main(String[] args) {
        try {
            // Public IP address of the server
            String ipServer = "2801:812:572:793:743f:8544:f9d4:45d9";
            int serverPort = 15000;

            // Connect to the server
            Socket socket = new Socket(ipServer, serverPort);
            System.out.println("Connected to the server");

            // Send and receive data
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            PrintWriter writer = new PrintWriter(socket.getOutputStream(), true);

            // Receive message from the server
            String message = reader.readLine();
            System.out.println("Message received:" + message);

            // Send message to the server
            writer.println("Hello from client");

            // Close socket
            socket.close();
        } catch (UnknownHostException e) {
            System.out.println("Error: Server not found");
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

compile the code in other PC:

javac ClientP2P.java

run the compiled code:

java ClientP2P

all working together:

---

(DDNS) dynamic url mapped to public ip ipv6

tcp.from-ar.com is mapped to an IPv4 and IPv6 with dyn. (works only with IPv6)

https://account.dyn.com

java code:

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

public class Client {
    public static void main(String[] args) {
        try {
            String domain = "tcp.from-ar.com";
            int serverPort = 12000;

            //Resolve domain name to IP address
            // Performs domain name resolution (DNS) to
            // obtain the IP address associated with the domain name tcp.from-ar.com.
            InetAddress[] addresses = InetAddress.getAllByName(domain);
            InetAddress iPv6Address = null;

            for (InetAddress address : addresses) {
                if (address instanceof Inet6Address) {
                    System.out.println("IPv6: " + address.getHostAddress());
                    iPv6Address = address;
                    break;
                }
            }

            // Create a socket that connects to the server
            Socket socket = new Socket();
            socket.setSoTimeout(30000); // 30 seconds
            socket.connect( new InetSocketAddress(iPv6Address, serverPort) );
            System.out.println("Connected to the server");

            // Send data to the server
            PrintWriter writer = new PrintWriter(socket.getOutputStream(), true);
            writer.println("Hello, server!");

            // Close the socket
            socket.close();
        } catch (UnknownHostException e) {
            System.out.println("The server could not be found");
        } catch (IOException e) {
            System.out.println("I/O Error: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

change tcp.from-ar.com for the real DDNS