DNS requests are cached by storing the locations (IP addresses) of pages you viewed. You may wish to flush (meaning clear, reset, or erase) a DNS cache, especially if you’re a systems administrator or a web developer, so that you have the latest DNS data. Resetting your DNS cache in Mac OS X is actually really easy.
If you are using Mac OS X v10.6, you can use the dscacheutil command. To do this, open Terminal and type:
sudo dscacheutil -flushcache
If you are using OS X Mountain Lion or Lion:
sudo killall -HUP mDNSResponder