Here is a demo of a Javascript designed to get the current IP address. A key element of the algorithm is that it makes a synchronous XMLHttpRequest to a server php script, i.e. ip = getIP(); does not return until the server has provided the answer. Here is a sample of the script in action:

<script language="javascript" src="get_ip.js"></script> <script> document.write('For example, your IP address is: ' + getIP()); </script>

It is also a useful basis for a method of avoiding those pesky accidental invalid clicks.