ICMP Port Unreachable是什么错误

我现在正在学习<<java网络编程》，书上有个例子，是做2个线程，一个发送udp包，另一个接收udp包，收发均使用同一个socket,但我一运行就提示：java.net.PortUnreachableException: ICMP Port Unreachable
发送线程类：
package test;

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

public class SenderThread extends Thread{
private InetAddress server;
private DatagramSocket socket;
private boolean stopped=false;
private int port;

public SenderThread(InetAddress server, int port) throws SocketException{
this.server = server;
this.port = port;
this.socket=new DatagramSocket();
this.socket.connect(server,port);
}

public void halt(){
this.stopped=true;
}

public DatagramSocket getSocket(){
return this.socket;
}

public void run() {

BufferedReader userInput=new BufferedReader(
new InputStreamReader(System.in)
);

try {
while(true){
if (stopped){
return;
}
String theLine=userInput.readLine();
if (theLine.equals(".")){
break;
}
byte[] data=theLine.getBytes();
DatagramPacket output=new DatagramPacket(data,data.length,server,port);
socket.send(output);
Thread.yield();
}
} catch (Exception e) {
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
}
}
}

接收线程类：
package test;

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

public class ReceiverThread extends Thread{
DatagramSocket socket;
private boolean stopped=false;

public ReceiverThread(DatagramSocket socket) {
this.socket = socket;
}

public void halt(){
this.stopped=true;
}

public void run() {
byte[] buffer=new byte[3000];
while (true){
if (stopped){
return;
}
DatagramPacket dp=new DatagramPacket(buffer,buffer.length);
try {
socket.receive(dp);//异常出现在这行。！！！！！！！！！！！！！！！！
String s=new String(dp.getData(),0,dp.getLength());
System.out.println(s);
Thread.yield();
} catch (Exception e) {
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
}
}
}
}

调用者：
package test;

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

public class UDPEchoClient {
public final static int DEFAULT_PORT=7;

public static void main(String[] args) {
try {
String hostname="localhost";
int port;

try {
port=Integer.parseInt(args[1]);
} catch (Exception e) {
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
port=DEFAULT_PORT;
}


if (args.length>0){
hostname=args[0];
}

InetAddress ia=InetAddress.getByName(hostname);
SenderThread sender=new SenderThread(ia,port);
sender.start();

Thread receiver=new ReceiverThread(sender.getSocket());
receiver.start();
} catch (UnknownHostException e) {
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
} catch (Exception e) {
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
}
}
}