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private void StartListening()
{
    //udp.Client.Bind();
    ar_ = udp.BeginReceive(Receive, new object());
    udp.Client.ReceiveTimeout = 50000;
    if (IPback.Length > 1)
        SendtoTable();
}

private void Receive(IAsyncResult ar)
{
    UdpClient client = new UdpClient();
    Messageback = "";
    IPback = "";

    IPEndPoint ip = new IPEndPoint(IPAddress.Broadcast, PORT_NUMBERReceive);

    byte[] bytes = udp.EndReceive(ar, ref ip);
    string message = Encoding.ASCII.GetString(bytes);
    Console.WriteLine("From {0} received: {1} ", ip.Address.ToString(), message);

    Messageback = message;
    IPback = ip.Address.ToString();

    StartListening();
}