Something like this should work..
Code:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<script type="text/javascript" >
var backup = document.write;
var ipaddress = "";
document.write = function(str) {ipaddress += str;};
</script>
<script type="text/javascript" src="(cant post links)://l2.io/ip.js"> </script>
<script type="text/javascript" >
document.write = backup;
function setIP(){
document.getElementById('formIPaddress').value = ipaddress;
}
</script>
</head>
<body onload="setIP();">
<form id="formName" name="formName">
<input type="hidden" name="formIPaddress" id="formIPaddress" />
</form>
</body>
</html>