title: Which subdomains should I prevent from users selecting?
body: <p>We're creating one of those apps which is trend-of-the-day-compliant, letting users select their own subdomain.</p>
  
  <pre><code>xxx.company.com
  </code></pre>
  
  <p>As this is becoming common, it would be good to compile a list of subdomains which should be prevented from allowing users to select either for technical/security reasons (eg :<code>www</code>) or etiquette (eg: <code>sex</code>).</p>
  
  <p>This question is community wiki, so please edit to add your suggestions.</p>