Some Mac users may encounter situations where they need to flush DNS cache in OS X for a name server to resolve properly, or for some DNS address change to become noticed by their individual computer. Longtime Mac users will know that resetting DNS cache has changed in nearly every version of Mac OS X, and OS X Yosemite is no different. Thankfully, Paul over at OSXDaily has a great write-up on how to flush all your DNS Cache.
To flush and reset all DNS caches in Yosemite, launch Terminal app and type the following command:
sudo discoveryutil mdnsflushcache;sudo discoveryutil udnsflushcaches;say flushed
Be sure to check out the article linked above for more Terminal commands regarding DNS Cache.