Are these Comcast DNS servers 68.87.76.182,68.87.76.134 down?

Dear Sir,

We had to change the DNS servers setting on one of our servers from 68.87.76.182,68.87.76.134 to 75.75.75.75,75.75.76.76 today to make name resolution work.  

What happened to 68.87.76.182,68.87.76.134 today?   No problem yesterday.

Also, ping to 68.87.76.182 and 68.87.76.134 failed.   No problem ping-ing to 75.75.75.75 and 75.75.76.76.

Has Comcast retired 68.87.76.182 and 68.87.76.134 today?   Or are they just temporarily down today?

Also, what is the correct Comcast DNS servers we should use now?

Thank you