Unload & Reload discoveryd to Fix DNS & Wi-Fi Failures in OS X Yosemite

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This trick involves refreshing the discoveryd service by unloading and reloading it with the launchctl command. 
This is a bit curious but apparently it works for some users, suggesting there could be an issue with discovery or resolving DNS on some Yosemite Macs. 
It’s certainly worth a try if the above tricks failed to resolve your wi-fi connectivity problems in OS X 10.10, as there are a fair amount of positive reports with this one:

Open Terminal (found in /Applications/Utilities/ or with Spotlight) and enter the following command:
`sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.discoveryd.plist`

Hit return and enter an admin password to use the sudo command
Now run the following command to reload discoveryd (this used to be called mDNSResponder)
`sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.discoveryd.plist`

Again hit Return to finish the command
You may need to relaunch apps that require network connectivity. Note that if you reboot the Mac with this one, you will have to repeat the above steps to unload and reload discoveryd into launchd.

http://osxdaily.com/2014/10/25/fix-wi-fi-problems-os-x-yosemite/

http://osxdaily.com/2014/11/20/flush-dns-cache-mac-os-x/