我使用多播制作了一个小聊天小程序。它可以在同一网络上的计算机之间正常工作，但如果计算机位于不同网络上则失败。为什么是这样？
import java.io.*;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class ClientA extends JApplet implements ActionListener, Runnable {
JTextField tf;
JTextArea ta;
MulticastSocket socket;
InetAddress group;
String name="";
public void start() {
try {
socket = new MulticastSocket(7777);
group = InetAddress.getByName("233.0.0.1");
socket.joinGroup(group);
socket.setTimeToLive(255);
Thread th = new Thread(this);
th.start();
name =JOptionPane.showInputDialog(null,"Please enter your name.","What is your name?",JOptionPane.PLAIN_MESSAGE);
tf.grabFocus();
}catch(Exception e) {e.printStackTrace();}
}
public void init() {
JPanel p = new JPanel(new BorderLayout());
ta = new JTextArea();
ta.setEditable(false);
ta.setLineWrap(true);
JScrollPane sp = new JScrollPane(ta);
p.add(sp,BorderLayout.CENTER);
JPanel p2 = new JPanel();
tf = new JTextField(30);
tf.addActionListener(this);
p2.add(tf);
JButton b = new JButton("Send");
b.addActionListener(this);
p2.add(b);
p.add(p2,BorderLayout.SOUTH);
add(p);
}
public void actionPerformed(ActionEvent ae) {
String message = name+":"+tf.getText();
tf.setText("");
tf.grabFocus();
byte[] buf = message.getBytes();
DatagramPacket packet = new DatagramPacket(buf,buf.length, group,7777);
try {
socket.send(packet);
}
catch(Exception e) {}
}
public void run() {
while(true) {
byte[] buf = new byte[256];
String received = "";
DatagramPacket packet = new DatagramPacket(buf, buf.length);
try {
socket.receive(packet);
received = new String(packet.getData()).trim();
}
catch(Exception e) {}
ta.append(received +"\n");
ta.setCaretPosition(ta.getDocument().getLength());
}
}
}