Listen from a different port SOCKET

I have the following program:

Server

    import java.io.IOException; 
import java.net.DatagramPacket; 
import java.net.DatagramSocket; 


public class Professor {


  public static void main(String[] args) throws IOException
    {
        try{
        DatagramSocket ds = new DatagramSocket(1234);
        byte[] receive = new byte[65535];
         System.out.println("Esperant rebré missatges");

        DatagramPacket DpReceive = null;
        {
            DpReceive = new DatagramPacket(receive, receive.length);
            ds.receive(DpReceive);

            System.out.println("Alumne:" + data(receive));
            System.out.println("");

            System.out.println("Professor: Missatge rebut.Enviat la resposta a l'alumne");     

        }
     } catch (IOException e) 
        {      
            e.printStackTrace();    
        }

    }
    public static StringBuilder data(byte[] a)
    {
        if (a == null)
            return null;
        StringBuilder ret = new StringBuilder();
        int i = 0;
        while (a[i] != 0)
        {
            ret.append((char) a[i]);
            i++;
        }
        return ret;
    }
}    

The client class.

   import java.io.DataOutputStream;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;


public class Alumne {

   public static void main(String args[]) throws IOException
    {
        try{   // Creant socket datagrama
        DatagramSocket ds = new DatagramSocket();
        InetAddress ip = InetAddress.getLocalHost();
        byte buf[] = null;

        {
            System.out.println("Comunicant al profesor que ja he acabat la practica..");
                  // Enviant missatge
            String inp="Ja tinc la practica acabada";
            System.out.println("");
            buf = inp.getBytes();
            DatagramPacket DpSend =
                  new DatagramPacket(buf, buf.length, ip, 1234);
            ds.send(DpSend);


            System.out.println("Esperant resposta del profesor....");
                // Rebent resposta



        }
    }     catch (IOException e) { 

      e.printStackTrace();  
}
    }
}

I have two doubts.

But the most important one is:

How can I make professor (server) use port 5555 And the "Student" client use port 5554 and they can be heard? I do not get it in any way.

thousand thanks!