OS X keeps a local cache of resolved DNS queries for a time defined by the DNS server. Sometimes it might be necessary to reset the cache immediately and re-query a DNS server. For example, you might need to do this if you are a network or server administrator and an entry on your DNS server has recently changed.

If your Mac isn’t using the latest DNS entries from your server, you can restart your Mac to update its cached information. If you need to update DNS entries on a server using OS X and you can’t restart the server, use the terminal commands below for the version of OS X you’re using.

This is also working for Yosemite (OS X v10.10.4 or later)


Source: https://support.apple.com/en-gb/HT202516