Can I open any TCP/UDP ports when using an Applet?

Yes, you can open any port for UDP provided there are no firewalls involved.

If firewalls are involved you would be prety much forced to either open the UDP port on the firewall or tunnel your packets over TCP port 80.

As far as a UDP server (with no firewall) is involved, your code would look like this:

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

// START main1
class EchoServer extends Thread {
    private DatagramSocket sock = null;
    EchoServer(int port) {
      try {
          sock = new DatagramSocket(port);
      } catch (SocketException e) {
          e.printStackTrace();
      }
    }
    public void run() {
      if (sock == null)
          return;

      byte[] inbuf = new byte[1024];
      DatagramPacket request = new DatagramPacket(inbuf, inbuf.length);
      try {
          while (true) {
            sock.receive(request);
            sock.send(request); // just return what was sent
          }
      } catch (IOException e) {
          e.printStackTrace();
      }
    }
}
// END main1

class Main {
    static int echo_port = 1257;

// START main2
public static void echo(String msg, InetAddress dst, int port) {
    byte[] inbuf = new byte[1024];  // default size
    byte[] outbuf = msg.getBytes();

    try {
      DatagramSocket client = new DatagramSocket(); // any port
      DatagramPacket request = new DatagramPacket(outbuf, outbuf.length,
          dst, port);
      DatagramPacket reply = new DatagramPacket(inbuf, inbuf.length);
      client.send(request);
      client.receive(reply);
      client.close();
      System.out.println(new String(reply.getData()));
    } catch (SocketException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
}
// END main2

    public static void main(String[] args) {
      if (args.length != 1) {
          System.err.println("Usage: java Main <message>");
          System.exit(1);
      }

      // start server
      EchoServer srv = new EchoServer(echo_port);
      srv.setDaemon(true);
      srv.start();

      try {
          String msg = "\n";
          InetAddress dst = InetAddress.getLocalHost();

          echo(args[0], dst, echo_port);

      } catch (UnknownHostException e) {
          System.err.println("Host not found: " + e);
      }

// START local
try {
    DatagramSocket client = new DatagramSocket();
    System.out.println("Using port number " + client.getLocalPort());
    System.out.println("Using address " + client.getLocalAddress());
// UNCOM      ...
    client.close();
} catch (SocketException e) {
    e.printStackTrace();
}
// END local
    }
}


The client would look like:

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

class Main {
    public static String dgExchange(String msg, InetAddress dst, int port) {
        byte[] outbuf = msg.getBytes();
        byte[] inbuf = new byte[256];  // default size

        try {
            // Send datagram
            DatagramPacket request =
                new DatagramPacket(outbuf, outbuf.length, dst, port);
            DatagramSocket sock = new DatagramSocket();
            sock.send(request);

            // Wait for reply
            DatagramPacket reply = new DatagramPacket(inbuf, inbuf.length);
            sock.receive(reply);

            System.out.println(
                "Received packet from:" + reply.getAddress() +
                " port: " + reply.getPort() +
                " length: " + reply.getLength());

            sock.close();
            return (new String(reply.getData()));
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return (null);
    }

    public static void main(String[] args) {
        try {
            String msg = "\n";
            int port = 13;
            InetAddress dst = InetAddress.getLocalHost();

            if (args.length > 0) {
                port = Integer.parseInt(args[0]);
                if (args.length >= 2)
                    msg = args[1];
                if (args.length == 3)
                    dst = InetAddress.getByName(args[2]);
            }

            System.out.println(dgExchange(msg, dst, port));
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
    }
}


Hope this helps.