DNS cache keeps an IP address stored in your computer. If you’re doing web development and you’ve moved your content to a new server sometimes you can’t reach it. The DNS entry needs to be changed.
In Mac OSX 10.10 Mavericks open up your terminal Application located in Applications/Utilities/Terminal
One Simple Step to Clear DNS Cache on OS X Yosemite:
sudo discoveryutil udnsflushcaches
Now, ping the domain that was cached and the new IP should show up for you.