Instructions on how to clear the DNS cache of various versions of MacOS

macOS 10.15 (Catalina) and later:

sudo dscacheutil - flushcache; sudo killall -HUP mDNSResponder

macOS 10.12 (Sierra), 10.13 (High Sierra), and 10.14 (Mojave):

sudo killall -HUP mDNSResponder

OS X 10.11 (El Capitan):

sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

OS X 10.10 (Yosemite):
For versions 10.10.4+, use:

sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

For versions 10.10.1, 10.10.2, and 10.10.3, use:

sudo discoveryutil mdnsflushcache; sudo discoveryutil udnsflushcache

OS X 10.9 (Mavericks):

dscacheutil - flushcache; sudo killall -HUP mDNSResponder

Mac OS X 10.7 (Lion) and 10.8 (Mountain Lion):

sudo killall -HUP mDNSResponder

Mac OS X 10.5 (Leopard) and 10.6 (Snow Leopard):

sudo dscacheutil -flushcache

Mac OS X 10.4 (Tiger):

lookupd -flushcache