router.jsx

<Router>
  <div>
    <ul>
      <li>
        <Link to="/">Home</Link>
      </li>
      <li>
        <Link to="/about">About</Link>
      </li>
      <li>
        <Link to="/topics">Topics</Link>
      </li>
    </ul>

    <hr />

    <Route exact path="/" component={Home} />
    <Route path="/about" component={About} />
    <Route path="/topics" component={Topics} />
  </div>
</Router>