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.
Open a Terminal Command
Step 2 – Type in the following command
sudo launchctl kickstart -k system/com.apple.networking.discoveryd
Try resolving or ping an address again.