Mac users need to run a quick Terminal command to flush the DNS cache, but the management differs depending on your version of macOS. First, press Command+Space to open Spotlight and search for "Terminal." Press Enter to open it.
Most modern versions of macOS—from OS X Lion through macOS Sierra—use the following command. Type it into the Terminal and press Enter:
sudo dscacheutil -flushcache;sudo killall -HUP mDNSResponder
If you're on OS X 10.10.1, 10.10.2, or 10.10.3, you'll need to run this command instead:
sudo discoveryutil udnsflushcaches;sudo discoveryutil
You won't see a success message for either command, but you can check the problematic website and see if it fixed the problem.