I am not getting any message from remote system in UDP java programming

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Java

UDP

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First program which runs on my computer

import java.io.*;

import java.net.*;

public class NewUDP22   /*main class*/
{
    public static void main(String arg[])throws Exception
    {  

      /*calling both threads ReceiveDataClass and SendDataClass*/

              ReceiveDataClass t=new ReceiveDataClass();
              SendDataClass obj=new SendDataClass();
              t.start();/*starting threads here*/

              obj.start();
    }   
}

class ReceiveDataClass extends Thread
{
    byte receiveData[]=new byte[10024];
    DatagramSocket ds;
    DatagramPacket dp;
    public void run()
    {
        try{ 
            while(true)
            {
                //byte receiveData[]=new byte[1024];

                ds=new DatagramSocket(50000);
                dp=new DatagramPacket(receiveData,receiveData.length);
                ds.receive(dp);

                String str=new String(dp.getData());
                System.out.println("person1:      "+str);

                ds.close();
            }
        }

        catch(Exception e){}
    }
}

class SendDataClass extends Thread
{
    public void run()
    {
        try
        {
            while(true)
            {
                byte sendData[]=new byte[100924];

                DatagramPacket dp;
                InetSocketAddress sd=new InetSocketAddress("192.168.8.101",40000);

                DatagramSocket ds=new DatagramSocket();
                BufferedReader dis;

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

                String data=dis.readLine();

                System.out.println("me:    "+data);

                sendData=data.getBytes();

                dp=new DatagramPacket(sendData,sendData.length,sd);

                ds.send(dp);

                ds.close();
            }
        }
        catch(IOException e){}
    }
}

Here is the second program which runs on different system

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

public class NewUDP2   /*main class*/
{
    public static void main(String arg[])throws Exception
    {  
      /*calling both threads ReceiveDataClass and SendDataClass*/
      ReceiveDataClass t=new ReceiveDataClass();

        SendDataClass obj=new SendDataClass();
        t.start();/*starting threads here*/
        obj.start();
    }   
}

class ReceiveDataClass extends Thread
{
    byte receiveData[]=new byte[10024];
    DatagramSocket ds;
    DatagramPacket dp;

    public void run()
    {
        try
        { 
            while(true)
            {
                //byte receiveData[]=new byte[1024];
                ds=new DatagramSocket(40000);
                dp=new DatagramPacket(receiveData,receiveData.length);
                ds.receive(dp);
                String str=new String(dp.getData());
                System.out.println("person1:      "+str);

                ds.close();
           }

        }catch(Exception e){}
    }
}

class SendDataClass extends Thread
{
    public void run()
    {
        try
        {
            while(true)
            {
                byte sendData[]=new byte[100924];
                DatagramPacket dp;


                InetSocketAddress  sd=new  InetSocketAddress("192.168.8.101",50000);

                DatagramSocket ds=new DatagramSocket();

                BufferedReader dis;

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

                String data=dis.readLine();

                System.out.println("me:    "+data);

                sendData=data.getBytes();

                dp=new DatagramPacket(sendData,sendData.length,sd);

                ds.send(dp);

                ds.close();

            }
        }catch(IOException e){}
    }

What I have tried:

When I run this both program on two different system then I can send the message on second system and He is also receiving the message. But when message is being sent by Remote System to my System then I am not able to get any message, even though message has sent by Remote System. I tried all ways which I could do. This is for a project in my university.