Can't access specific websites with Macbook Air (10.7.5 Lion)
For such problem one more option is there where you can check your DNS settings. 
To fix that problem, open the Network pane of System Preferences and select your network connection in the list on the left. Click Advanced followed by DNS. 

In the DNS Servers field, you should see one or more IP addresses. If those addresses are enabled (black, as opposed to gray), select each one in turn and click the minus-sign (-) button. Then, regardless of whether there are already addresses there in gray, click the plus-sign (+) button and enter 208.67.222.220; repeat with 208.67.222.222.

Some more Ipv4 and Ipv6 addresses are given at computerknow.org, you may see there.