pokk,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<input id="id_IP" class="act">
<script>
var test="\"IP\":\"192.168.1.4\",\"Mask\":\"255.255.255.0\"";
var jsonList = document.querySelectorAll('.act'); //JSON act
var objJSON = eval('({' + test + '})');
[].forEach.call(jsonList , function(el) {
var Key = el.id.slice(3);
if(Key in objJSON) el.value=objJSON[Key]
});
</script>
</body>
</html>