From a hints.macworld.com article, Aug 08, 2012 07:30AM • Contributed by kirkmc.
I came across this Apple technical note this morning, which describes how to flush the DNS cache on your Mac…
Go there for more details, or grab the pertinent info here:
OS X 10.5 & 10.6: sudo dscacheutil -flushcache
OS X 10.7 & 10.8: sudo killall -HUP mDNSResponderby