thanks you seemed to have prooved that a regular applet can receive
udp datagram from its server though all that remains mysterious and my
server does not work:

Facts:
- There is a firewall on my server side and the windows xp firewall
here on my desktop
- your udp applet http://www.knutejohnson.com/echo.html works fine
from  my desktop
- your udp applet does not work with my c# echo udp (I plugged it to
my server)- server receives packets and echo them back but they nerver
reach back the applet
-my c# echo udp works fine with a standalone java app from my local
computer
-my applet (which does pretty much the same than knute's one) does not
work with my c# echo udp server - - server receive packets and echo
them back but they nerver reach back the applet
- if my applet is signed then it works fine with my c# echo udp server

I need this applet working without being signed !!
Any clue ??
thanks

here is new version of my c# echo udp server :

  protected void StartUDP()
   {
       Socket soUDP = new Socket(AddressFamily.InterNetwork,
              SocketType.Dgram, ProtocolType.Udp);
       try
       {

           //60 sec timeout
           soUDP.SetSocketOption(SocketOptionLevel.Socket,
SocketOptionName.ReceiveTimeout, 3000);

           // On doit lier la Socket à un port d'écoute sur la
machine. On choisit également de se mettre en écoute sur toutes les
interfaces réseaux présentes (IPAddress.Any).
           soUDP.Bind(new IPEndPoint(IPAddress.Any,
(int)Application["port"]));
           Application["log"] = Application["log"] + "isBinded<br>";
           Application["isStarted"] = true;
           Application["serverStatus"] = "UDP server isStarted at " +
DateTime.Now + "<br>";
           Application["log"] = Application["log"] + "start waiting
at " + DateTime.Now + "<br>";

           while (!((Boolean)(Application["stopRequested"])))
           {
               //if datagram receveided is bigger than 12 exception
will be thrown on receive
               Byte[] received = new Byte[12];

               // Empty endpoint
               EndPoint remoteEP = new IPEndPoint(IPAddress.Any, 0);
               try
               {

                   //remoteEP will be instanciated by the method
                   // with remote sender info
                   int bytesReceived = soUDP.ReceiveFrom(received,
ref remoteEP);
                   //echoes back
                   soUDP.SendTo(received, remoteEP);

                   String dataReceived =
System.Text.Encoding.ASCII.GetString(received);
                   IPEndPoint ip = (IPEndPoint)remoteEP;
                   Application["log"] = Application["log"] +
"Datagram packet received from "+ ip.Address +":"+ip.Port+ "<br>";
                   Application["log"] = Application["log"] +
"Datagram packet length=" + bytesReceived + " dataReceived=" +
dataReceived + "<br>";
                   Application["log"] = Application["log"] + "Echoed
it back to " + ip.Address +":"+ip.Port+ "<br>";

               }
               catch (Exception t)
               {
                   // Here it is a timeout (but not a
sockettimeoutexception)
                  // Application["log"] = Application["log"] + "" +
t.Message;
               }

           }
       }
       catch (Exception e)
       {
           Application["serverStatus"] = "UDP server probably stoped
on exception at " + DateTime.Now + "<br>";
           Application["isStarted"] = false;
           Application["log"] = Application["log"] + "Exception at "
+ DateTime.Now + "<br>";
           Application["log"] = Application["log"] + e.Message +
"<br>";
           soUDP.Close();
       }

           Application["isStarted"] = false;
           Application["log"] = Application["log"] + "finally closing
server";
           Application["serverStatus"] = "UDP server stoped at " +
DateTime.Now + "<br>";
           soUDP.Close();

   }