AT&T did not respond to several requests for comment.

DNS is responsible for converting human-friendly domain names into the numeric IP (Internet protocol) addresses that computers use to route data. When it fails, computers are unable to route data to its intended destination, even though the destination server remains online and accessible.

"We got our first report of problems at 6:31am Pacific time," said Daniel Blackmon, director of software development, at Worldwide Environmental Products. The company tests vehicle emissions and has remote units deployed that report back to central servers.

"The problems mean none of the equipment we have in the field can contact our servers, and there is a limit to the amount of information they can hold offline."