title: What are the possible return values for $_SERVER['REMOTE_ADDR'];?
body: <p>In writing a login module, I want to log IP's as an additional measure for verifying who's on the other side is still the same person on the other side.</p>
  
  <p>I'm using <code>$_SERVER['REMOTE_ADDR']</code> as one (of many) ways to get the remote machine's IP address.  Aside from an IPv4 or IPv6 address, are there any other values i should expect this to return?</p>