HTML

NavBar Bootstrap v4 Fix

<nav class="navbar navbar-expand-sm navbar-dark fixed-top bg-dark">
  <div class="container">
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    </button>
    <a class="navbar-brand" href="/">Application name</a>
    <div class="navbar-collapse collapse" id="navbarSupportedContent">
      <ul class="nav navbar-nav mr-auto">
        <li class="nav-item"><a href="/" class="nav-link">Home</a></li>
        <li class="nav-item"><a href="/Home/About" class="nav-link">About</a></li>
        <li class="nav-item"><a href="/Home/Contact" class="nav-link">Contact</a></li>
      </ul>

      <ul class="nav navbar-nav navbar-right mr-auto">
        <li class="nav-item"><a href="/Account/Register" id="registerLink" class="nav-link">Register</a></li>
        <li class="nav-item"><a href="/Account/Login" id="loginLink" class="nav-link">Log in</a></li>
      </ul>

    </div>
  </div>
</nav>

fixed bootstrap navbar

<nav class="navbar fixed-top navbar-light bg-light">
  <a class="navbar-brand" href="#">Fixed top</a>
</nav>

bootstrap navbar fixed top

<section id="headnev" class="navbar navbar-fixed-top">  

how to fix navbar in bootstrap

<nav className="fixed-top">