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for fixed height: http://jsfiddle.net/CriddleCraddle/Wj9dD/

dynamic height:
http://jsfiddle.net/shubhampatwa/46ovg69z/


HTML:

<nav class="navbar navbar-default" id="mainnav">
</nav>


<script>
$(document).ready(function() {
  // get current top offset 
  var navpos = $('#mainnav').offset(); 
  console.log(navpos.top);
    $(window).bind('scroll', function() {
      if ($(window).scrollTop() > navpos.top) {
        // add fixed top class to make nav bar fixed
       $('#mainnav').addClass('navbar-fixed-top');
       }
       else {  // remove fixed class when window > navbar position top
         $('#mainnav').removeClass('navbar-fixed-top');
       }
    });
});


</script>



For Mobile devices:
<script>
var newWindowWidth = $(window).width();
if (newWindowWidth < 481) {
  //Place code inside it...
}

</script>