How can I diagnose why I can no longer access certain sites on certain networks since I upgraded to Mountain Lion?
Solution 1:
When I've had this issue in the past, like the two day period when I couldn't access Flickr from my laptop but I could from other machines, the solution was to flush the DNS Cache: essentially a small database on your machine that can help (or so my understanding) speed up network access.
According to the latest Apple Support document on this, the command is:
sudo killall -HUP mDNSResponder
http://support.apple.com/kb/HT5343