title: setsockopt() SOL_IP API level and IPT_SO_SET_REPLACE switch
body: <p>What exactly does <code>SOL_IP</code> mean as the API level of the <code>setsockopt</code> function
  and what exactly does the <code>IPT_SO_SET_REPLACE</code> switch do?</p>
  
  <p>I tried to search for both in Google but I found nothing.<br>
  Please help me understand them (if you can expand and explain with examples I'd really appreciate it)</p>