Step 1. Open the Terminal.app, either by starting typing Terminal on the Spotlight or by going into Applications.
Step 2. Go to Utilities and click Terminal:
Step 3. Run one of the following commands depending on the version of OS X you're using and press Enter, then type your user password when prompted:

OS X El Capitan:

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sudo dscacheutil -flushcache;sudo killall -HUP mDNSResponder;echo flushed
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OS X Yosemite:

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sudo killall -HUP mDNSResponder
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OS X Mavericks, Mountain Lion, and Lion:

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sudo killall -HUP mDNSResponder
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Mac OS X v10.6:

sudo dscacheutil -flushcache