The code below shows how to get the host from where the applet was loaded:

Java Code:
public class FromWhere extends Applet {
  public void init() {
    Label label = new Label(getCodeBase().toString());

    // you can check the value of getCodeBase()
    // to implements a simple copy protection
    // scheme. If it's not equals to your
    // URL then quit.
    // NOTE : This is not a very strong protection!