The code below shows how to get the host from where the applet was loaded:
public class FromWhere extends Applet {
public void init() {
Label label = new Label(getCodeBase().toString());
add(label);
//
// 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!
//
}
}