title: ssh, Protocol major versions differ
body: <p>It worked when I use Mac OS X El Captain, but after I update to MacOS Sierra, it fails:</p>
  
  <pre><code>$ ssh -1 $IP
  ssh1 is not supported
  $ ssh -2 $IP
  Protocol major versions differ: 2 vs. 1
  </code></pre>
  
  <p>So which version should I use?</p>