I was having a little trouble with internet access yesterday, so the first thing I usually do is ping something like google.com (they seem to be always up). If the response comes back, I at least know that I'm getting to the DNS server and do have some sort of connection.

One site in particular seemed to be having trouble - couldn't connect etc., so when I pinged it, the requests timed out. As expected.

However, today, I was just pinging randomly, and flyertalk.com was not responding. Since it was patently up and on line, I was wondering if it's optional to respond to pings?