With the recent update of OS X Yosemite to 10.10.4, the usual way of flushing the DNS using discoveryutil no longer works.
$ sudo discoveryutil mdnsflushcache; sudo discoveryutil udnsflushcaches sudo: discoveryutil: command not found sudo: discoveryutil: command not found
discoveryutil has been replaced by mDNSResponder in 10.10.4.
The following command should clear the DNS cache.
sudo killall -HUP mDNSResponder