Edit in JSFiddle

<!-- Navigation -->
<nav class="navbar navbar-expand-lg navbar-light bg-light 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>

<header>
  <div id="carouselExampleIndicators" class="carousel slide" data-ride="carousel">
    <ol class="carousel-indicators">
      <li data-target="#carouselExampleIndicators" data-slide-to="0" class="active"></li>
      <li data-target="#carouselExampleIndicators" data-slide-to="1"></li>
      <li data-target="#carouselExampleIndicators" data-slide-to="2"></li>
    </ol>
    <div class="carousel-inner" role="listbox">
      <!-- Slide One - Set the background image for this slide in the line below -->
      <div class="carousel-item active" style="background-image: url('https://source.unsplash.com/LAaSoL0LrYs/1920x1080')">
        <div class="carousel-caption d-none d-md-block">
          <h2 class="display-4">First Slide</h2>
          <p class="lead">This is a description for the first slide.</p>
        </div>
      </div>
      <!-- Slide Two - Set the background image for this slide in the line below -->
      <div class="carousel-item" style="background-image: url('https://source.unsplash.com/bF2vsubyHcQ/1920x1080')">
        <div class="carousel-caption d-none d-md-block">
          <h2 class="display-4">Second Slide</h2>
          <p class="lead">This is a description for the second slide.</p>
        </div>
      </div>
      <!-- Slide Three - Set the background image for this slide in the line below -->
      <div class="carousel-item" style="background-image: url('https://source.unsplash.com/szFUQoyvrxM/1920x1080')">
        <div class="carousel-caption d-none d-md-block">
          <h2 class="display-4">Third Slide</h2>
          <p class="lead">This is a description for the third slide.</p>
        </div>
      </div>
    </div>
    <a class="carousel-control-prev" href="#carouselExampleIndicators" role="button" data-slide="prev">
          <span class="carousel-control-prev-icon" aria-hidden="true"></span>
          <span class="sr-only">Previous</span>
        </a>
    <a class="carousel-control-next" href="#carouselExampleIndicators" role="button" data-slide="next">
          <span class="carousel-control-next-icon" aria-hidden="true"></span>
          <span class="sr-only">Next</span>
        </a>
  </div>
</header>

<!-- Page Content -->
<section class="py-5">
  <div class="container">
    <h1 class="display-4">Full Page Image Slider</h1>
    <p class="lead">The background images for the slider are set directly in the HTML using inline CSS. The images in this snippet are from <a href="https://unsplash.com">Unsplash</a>, taken by <a href="https://unsplash.com/@joannakosinska">Joanna Kosinska</a>!</p>
  </div>
</section>
.carousel-item {
  height: 100vh;
  min-height: 350px;
  background: no-repeat center center scroll;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

              

External resources loaded into this fiddle: