There appears to be a bug in OSX Yosemite 10.10 which is causing the ping and nslookup command to return the error  Cannot Resolve xxxxxxx.com : Unknown Host.  Even when rebooting it doesn’t resolve the issue.

To fix this please try restarting discoveryd.

Step 1

Open a Terminal Command

Step 2 – Type in the following command

sudo launchctl kickstart -k system/com.apple.networking.discoveryd

Step 3

Try resolving or ping an address again.