官网的面包屑navbar导航都是左右两边都是靠边的，下面展示改进之后导航居中，大屏幕是.container效果，移动端是.container-fluid效果：
这个导航还有一个常用的下拉Dropdown
<!DOCTYPE doctype html>
<html lang="en">
<head>
<!-- Required meta tags -->
<meta charset="utf-8">
<meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<link href="images/lx.ico" rel="shortcut icon">
<!-- Bootstrap CSS -->
<link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/4.0.0/css/bootstrap.min.css">
<link rel="stylesheet" href="https://cdn.bootcss.com/minireset.css/0.0.2/minireset.min.css" rel="stylesheet">
<title>
myBootstrap-navbar
</title>
<style>
.container{
padding-right: 0;
padding-left: 0;
}
@media (max-width: 1200px) {
.container {
max-width: 100%;
}
}
</style>
</head>
<body>
<div class="bg-light">
<div class="container">
<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a class="navbar-brand" href="#">Navbar</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<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 dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">Disabled</a>
</li>
</ul>
<form class="form-inline my-2 my-lg-0">
<input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
</form>
</div>
</nav>
</div>
</div>
<script src="https://cdn.bootcss.com/jquery/3.3.1/jquery.slim.min.js"></script>
<script src="https://cdn.bootcss.com/popper.js/1.12.9/umd/popper.min.js"></script>
<script src="https://cdn.bootcss.com/twitter-bootstrap/4.2.1/js/bootstrap.min.js"></script>
<!-- 各种库可以在bootcdn上下载到本地https://www.bootcdn.cn/ -->
</body>
</html>
宽屏时候，导航居中
wide screen
移动端的时候，导航占满整个屏幕，并显示面包屑
narrow screen
