Mountain Lion upgrade and many sites are not opening

I've upgraded to Mountain Lion and since then many sites including Google (and all of its related services), facebook and many other sites are not opening. i can't even open gmail.com. the name resolutions seem to be working fine.

dig gmail.com

; <<>> DiG 9.8.1-P1 <<>> gmail.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24840 ;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION: ;gmail.com. IN A

;; ANSWER SECTION: gmail.com. 300 IN A 209.85.175.17 gmail.com. 300 IN A 209.85.175.19 gmail.com. 300 IN A 209.85.175.83 gmail.com. 300 IN A 209.85.175.18

;; Query time: 262 msec ;; SERVER: 8.8.8.8#53(8.8.8.8) ;; WHEN: Sat Jul 28 11:44:03 2012 ;; MSG SIZE rcvd: 91

In the status bar, it says Waiting for domain. and later it shows errors like EMPTY RESPONSE, TIMED OUT etc.

i even can't open yahoo, twitter. i've tested this from Chrome, Safari, Firefox. But all these sites open from my wife's macbook (snow leopard) that is using same internet connection.

however, i can open many sites including all SE sites, github and many others.

is there anything i can do? as i can't google, i'm literally can't search web too :(.

thanks in advance.

This issue affected several people after upgrading to Mountain Lion. For many, reducing their MTU size solved the issue:

1. Open System Preferences.
2. Select Network.
3. Select your connection and choose Advanced...
4. Select Hardware.
5. Change Configuration to Manually
6. Reduce the MTU size to 1300. Most people had good experiences with this value.
7. Observe how ping behaves over time. You can further reduce the MTU size in steps of 4.

However, there seems to be another issue: Proxies. The solution suggested here is:

1. Open System Preferences.
2. Select Network.
3. Select your connection and choose Advanced...
4. Select Proxies.
5. Turn off Auto Discovery Proxies.
6. Apply the settings.

This should be fixed with an upgrade to OS X 10.8.1 (not sure if an upgrade to Mountain Lion directly brings you to the latest upgrade, or to 10.8.0).

Actually, I would first try the upgrade to 10.8.1. If this does not work, try the proxy solution. If this does not work, try the MTU size reduction.