ブートストラップcssはnavbar navbar-fixed-topの下のコンテナの一部を非表示にします

var onResize = function() {
  // apply dynamic padding at the top of the body according to the fixed navbar height
  $("body").css("padding-top", $(".navbar-fixed-top").height());
};

// attach the function to the window resize event
$(window).resize(onResize);

// call it also when the page is ready after load or reload
$(function() {
  onResize();
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>



   <div class="container-fluid">
        <nav class="navbar navbar-default navbar-fixed-top">
    
            <div class="navbar-header">
                <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
                    <span class="sr-only">Toggle navigation</span>
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                </button>
                <a class="navbar-brand" href="#">Bootstrap</a>
            </div>
            <div id="navbar" class="navbar-collapse collapse">
                <ul class="nav navbar-nav">
                    <li class="active"><a href="#">Home</a></li>
                    <li class="dropdown">
                            <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
                            <ul class="dropdown-menu">
                                <li><a href="#">Action</a></li>
                                <li><a href="#">Another action</a></li>
                                <li><a href="#">Something else here</a></li>
                                <li role="separator" class="divider"></li>
                                <li class="dropdown-header">Nav header</li>
                                <li><a href="#">Separated link</a></li>
                                <li><a href="#">One more separated link</a></li>
                            </ul>
                        </li>
                </ul>
                <div class="navbar-right">
                </div>
            </div>
        </nav>
    </div>
    <div class="page-header">
        <div class="clearfix">
            <div class="col-md-12">
                <div class="col-md-8 col-sm-6 col-xs-12">
                    <h1>Registration form <br /><small>A Bootstrap template showing a registration form with standard fields</small></h1>
                </div>
            </div>
        </div>
    </div>
        <div class="container">
        <div class="row">
            <form role="form">
                <div class="col-lg-6">
                    <div class="well well-sm"><strong><span class="glyphicon glyphicon-asterisk"></span>Required Field</strong></div>
                    <div class="form-group">
                        <label for="InputName">Enter Name</label>
                        <div class="input-group">
                            <input type="text" class="form-control" name="InputName" id="InputName" placeholder="Enter Name" required>
                            <span class="input-group-addon"><span class="glyphicon glyphicon-asterisk"></span></span>
                        </div>
                    </div>
                    <div class="form-group">
                        <label for="InputEmail">Enter Email</label>
                        <div class="input-group">
                            <input type="email" class="form-control" id="InputEmailFirst" name="InputEmail" placeholder="Enter Email" required>
                            <span class="input-group-addon"><span class="glyphicon glyphicon-asterisk"></span></span>
                        </div>
                    </div>
                    <div class="form-group">
                        <label for="InputEmail">Confirm Email</label>
                        <div class="input-group">
                            <input type="email" class="form-control" id="InputEmailSecond" name="InputEmail" placeholder="Confirm Email" required>
                            <span class="input-group-addon"><span class="glyphicon glyphicon-asterisk"></span></span>
                        </div>
                    </div>
                    <div class="form-group">
                        <label for="InputMessage">Enter Message</label>
                        <div class="input-group">
                            <textarea name="InputMessage" id="InputMessage" class="form-control" rows="5" required></textarea>
                            <span class="input-group-addon"><span class="glyphicon glyphicon-asterisk"></span></span>
                        </div>
                    </div>
                    <input type="submit" name="submit" id="submit" value="Submit" class="btn btn-info pull-right">
                </div>
            </form>
    </div>

<script type="text/javascript">
$(document).ready(function(e) {
   var h = $('nav').height() + 20;
   $('body').animate({ paddingTop: h });
});
</script>