Access denied while calling external URL ajax call

I got the access denied error when i am calling external URL from Ajax call. it is working fine without SharePoint 2010.

 <script>
    function httpGet(theUrl) {
        debugger;
        var xmlHttp = null;
        xmlHttp = new XMLHttpRequest();
        xmlHttp.open("GET","http://ip.jsontest.com/", false);
        xmlHttp.send(null);
        alert(xmlHttp.responseText);
        var obj = JSON.parse(xmlHttp.responseText);
        alert(obj.ip);
        return xmlHttp.responseText;
    }
</script>

Please help me to resolve this.