The Mac equivalent of ipconfig /flushdns in Windows is as follows.
In Mavericks (OSX 10.9):
dscacheutil -flushcache; sudo killall -HUP mDNSResponder
In Lion (OSX 10.7) and Mountain Lion (OSX 10.8):
sudo killall -HUP mDNSResponder
In OSX 10.5 and 10.6
dscacheutil -flushcache
