I have got the sollution
<script type="text/javascript">
$.getJSON("http://jsonip.appspot.com?callback=?",
function setvar(data) {
var a = data.ip;
var hiddenControl = '<%= ipHide.ClientID %>';
document.getElementById(hiddenControl).value = a;
alert(document.getElementById(hiddenControl).value);
});
</script>
In form tag
<input id="ipHide" type="hidden" runat="server" />