You can think of it this way: imagine you are on a vacation abroad, and you want to write a letter to your relatives back home. After you finished writing the letter, you would write on the envelope, “my home” as the target address. The mailman will not understand where to send the letter, of course. The mailman has his own idea of “my home”, but it is not your home, but rather, it’s his home. He will need your public home address in order to send the letter to your relatives. This scenario is equivalent to asking Loadmill (the mailman) to send an HTTP request (the letter) to localhost (“my home”).