Microsoft PC:
If you are running a Microsoft Driven PC then you need to open a Command Prompt Window and run the following command:
ipconfig /flushdns
If you are running a Mac:
OS X Mountain Lion or Lion
Use the following Terminal command to reset the DNS cache:
sudo killall -HUP mDNSResponder
Mac OS X v10.6
Use the following Terminal command to reset the DNS cache:
sudo dscacheutil -flushcache