macOS 10.10.4 – 10.14.x
sudo killall -HUP mDNSResponder
macOS 10.10.3
sudo discoveryutil mdnsflushcache
OS X 10.7 – 10.8
sudo killall -HUP mDNSResponder
OSX 10.5 – 10.6
sudo dscacheutil -flushcache
Windows
ipconfig /flushdns
Linux
/etc/init.d/named restart
or…
/etc/init.d/nscd restart