How can I find out who is accessing my server ?

If you're using a DatagramSocket, every packet that you receive will contain the address and port from which it was sent.

Example

DatagramPacket packet = null;

// Receive next packet

myDatagramSocket.receive(packet);

// Print address + port

System.out.println ("Packet from : " + 

     packet.getAddress().getHostAddress() + ':' + packet.getPort());

If you're using a ServerSocket, then every socket connection you accept will contain similar information.

Example

Socket mySock = myServerSocket.accept();

// Print address + port

System.out.println ("Connection from : " + mySock.getInetAddress().getHostAddress() + ':' + mySock.getPort());