Yosemite DNS "cannot resolve host"
Usually it is recommended to use public DNS servers like Google 8.8.8.8 and 8.8.4.4 in situation like this.

You can delete any other DNS in your syst pref Networks- DNS or make sure the Google is at the top of the list.

If you want to get in to the DNS then use a little app called namebench which will analyze and recommend to you the best (fastest) DNS server. I have used the namebench and followed its recommendation and yes, my loading of the web pages is faster.