title: tcpdump -n 'tcp[13] & 16 !=0' | wc -l v.s. tcpdump -n 'tcp[tcpflags] = tcp-ack' | wc -l ==> different results?
body: <p>I'm trying to understand the following;</p>
  <p>When trying <code>tcpdump -n 'tcp[13] &amp; 16 !=0' | wc -l</code> on a file, I get a different result (27833) than when I try <code>tcpdump -n 'tcp[tcpflags] == tcp-ack' | wc -l</code> (16433) on that same file.</p>
  <p>I was hoping someone could explain me the difference.</p>