Edit in JSFiddle

<!-- Navigation -->
<nav class="navbar navbar-expand-lg navbar-dark bg-dark fixed-top">
  <div class="container">
    <a class="navbar-brand" href="#">Start Bootstrap</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarResponsive" aria-controls="navbarResponsive" aria-expanded="false" aria-label="Toggle navigation">
          <span class="navbar-toggler-icon"></span>
        </button>
    <div class="collapse navbar-collapse" id="navbarResponsive">
      <ul class="navbar-nav ml-auto">
        <li class="nav-item active">
          <a class="nav-link" href="#">Home
                <span class="sr-only">(current)</span>
              </a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="#">About</a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="#">Services</a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="#">Contact</a>
        </li>
      </ul>
    </div>
  </div>
</nav>

<!-- Full Page Image Header with Vertically Centered Content -->
<header class="masthead">
  <div class="container h-100">
    <div class="row h-100 align-items-center">
      <div class="col-12 text-center">
        <h1 class="m-0 display-4">Vertically Centered Masthead Content</h1>
      </div>
    </div>
  </div>
</header>

<!-- Page Content -->
<section class="py-5">
  <div class="container">
    <h2>Page Content</h2>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Repellendus ab nulla dolorum autem nisi officiis blanditiis voluptatem hic, assumenda aspernatur facere ipsam nemo ratione cumque magnam enim fugiat reprehenderit expedita.</p>
  </div>
</section>
.masthead {
  height: 100vh;
  min-height: 500px;
  background-image: url('https://source.unsplash.com/BtbjCFUvBXs/1920x1080');
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
}

              

External resources loaded into this fiddle: