Windows
Go to Start > Run
Type "cmd" and hit enter
Type "ipconfig /flushdns" and hit enter
Mac
Open the Terminal application. This is normally at Mac HD > Applications > Utilities
For Lion and newer:
Type "sudo killall -HUP mDNSResponder" and hit enter. It will ask for your password.
For Leopard or Snow Leopard:
Type "dscacheutil -flushdns" and hit enter
Before Leopard:
Type "lookupd -flushcache" and hit enter
Linux
Restart the nscd daemon by typing "/etc/init.d/nscd restart" and hitting enter