Please read this whole message and be sure you understand all of it before doing anything. Back up all data before making changes to your settings.
Write down the server addresses.
Click Cancel to close the Advanced sheet. Unlock the preference pane, if necessary, by clicking the lock icon in the lower left corner and entering your password. Open the DNS tab again and change the server addresses to the following:
That's Google DNS. Click OK, then Apply.
In Safari, select
Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data
and confirm. If you’re using another browser, empty the cache. Test. Any difference?
1. If you lose Internet access after making the above change to your network settings, delete the Google servers in the Network preference pane, then select the TCP/IP tab and click Renew DHCP Lease. That should restore the original DNS settings; otherwise restore them yourself.
2. I’m not advocating Google or anything else as a DNS provider; the server addresses are offerred merely for testing purposes. There may be privacy and technical issues involved in using that service, which you should investigate personally before you decide whether to keep the settings. Other public DNS services exist.