<nav class="navbar navbar-expand-sm navbar-dark bg-dark">
    <a class="navbar-brand" href="#">Expand at sm</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarsExample03" aria-controls="navbarsExample03" aria-expanded="false" aria-label="Toggle navigation">
      <span class="navbar-toggler-icon"></span>
    </button>

    <div class="collapse navbar-collapse" id="navbarsExample03">
      <ul class="navbar-nav mr-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="#">Link</a>
        </li>
        <li class="nav-item">
          <a class="nav-link disabled" href="#">Disabled</a>
        </li>
        <li class="nav-item dropdown">
          <a class="nav-link dropdown-toggle" href="#" id="dropdown03" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dropdown</a>
          <div class="dropdown-menu" aria-labelledby="dropdown03">
            <a class="dropdown-item" href="#">Action</a>
            <a class="dropdown-item" href="#">Another action</a>
            <a class="dropdown-item" href="#">Something else here</a>
          </div>
        </li>
      </ul>
      <form class="form-inline my-2 my-md-0">
        <input class="form-control" type="text" placeholder="Search">
      </form>
    </div>
  </nav>