title: listen to specific port no working on nginx
body: <p>The OS is Centos 7.3 , firewall is closed.
  The server config on nginx is like.</p>
  
  <pre><code>server {
      listen       2001;
      listen       [::]:2001;
      server_name  _;
      root  /usr/share/nginx/html;
      index index.html;
  }
  </code></pre>
  
  <p>I don't have a domain now, so the ip:port is what i want.
  but when I typed xxx.xxx.xxx.xxx:2001 on the browser, can't access the page.
  But the 80 port works fine.</p>
  
  <p>I'm a new guy to both linux and nginx, any clue will help.</p>