How can I diagnose why I can no longer access certain sites on certain networks since I upgraded to Mountain Lion? Since upgrading to Mountain Lion, I can no longer see certain sites, when connected to certain networks.

I have a 2011 15" MacBook Pro (Quad-Core i7) which came with Lion (actually I think it came with Snow Leopard and a free upgrade to get Lion form the Mac App Store).

I work as a consultant on a client site. I connect my laptop to a WiFi network at the client.

I have recently upgraded to Mountain Lion, and since then when I am on the client's network, I can no longer visit certain sites. One of the sites affected is *.apple.com - including www.apple.com, the iTunes store, the App Store and developer.apple.com.

I used to be able to access all of these sites prior to upgrading.

I thought that maybe the client's IT services had blocked certain sites at the same time - maybe seeing a spike of traffic to the app store when people upgraded, but that is not the case. I can access the site from other devices on the same network. Also, apple.com is not the only one I cannot access.

I thought that maybe there was something wrong with my laptop, but I have no trouble connecting to these sites when I am connected to my own company's VPN (over the very same client's WiFi network).

As far as I can tell, it only happens for certain sites, when connected through this one network, only after I upgraded to Mountain Lion.

I suspect that it has something to do with DNS resolution, and possibly to do with sites that redirect in a certain way - for example apple.com redirects to edge networks like Akamai - and possibly something to do with how the client network is set up, and possibly to do with some change in the network stack in Mountain Lion (but none of these factors on its own - only in combination!)

Has anyone else experienced this? 

What can I do to fix it? 

Or at the very least to help diagnose it and report to the company's IT team and to Apple?

Update: I have noticed, since I posted this, that it only affects Safari, iTunes Store and Mac App Store (I imagine they all use the same underlying component), but not Firefox or Chrome.

UPDATE 2: Since the 10.8.2 update to Mountain Lion, this problem has gone away – Vihung Oct 8 '12
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