Internet advertising company DoubleClick Inc. was shut down Tuesday by a denial of service (DoS) attack launched from computers on the Internet, a company spokesperson confirmed.

The massive attack began at around 10:30 a.m. Eastern Time and crippled the company’s Web site and its advertising servers, which distribute Web advertisements to other Web sites on the Internet. Ripple effects from the attacks were felt across the Internet, as Web pages that display DoubleClick ads struggled to retrieve them from the company’s servers, causing “severe disruption” for DoubleClick customers, according to a company statement.

Leading Web sites all experienced significant slow downs during the period covered by the attack, including Web pages for the Washingtom Post Co., New York Times Co., Cnet Networks Inc. and Nortel Networks Corp. according to Keynote Systems Inc., a Web performance measurement company in San Mateo, Calif.

Keynote measurements for the period covered by the attacks show that the “base page” — or basic Hypertext Markup Language (HTML) documents — served by those Web sites loaded quickly, but that the “full page,” which includes any content the Web page points to, suddenly began to load very slowly, said Lloyd Taylor, vice-president of operations at Keynote.

DoubleClick’s domain name system (DNS) servers were the target of the attack, which came from “outside sources” that it has not identified and lasted for approximately four hours, said Jennifer Blum, vice-president of corporate communications at DoubleClick.

DNS is the system of servers that matches up reader-friendly names such as with the numeric Internet Protocol (IP) addresses used by machines on the Internet to route traffic.

Keynote recorded a threefold slowdown in response time for Web pages beginning at around 7:00 a.m. Eastern time and ending at 1:30 p.m. Eastern Time. The company does not know what the source of the slowdowns was, but the behavior of the pages is consistent with a DoS attack on DoubleClick, Taylor said.

The performance of DoubleClick’s servers had returned to normal by late Tuesday afternoon, East Coast time, Taylor said.

The situation at DoubleClick has improved over the last few hours and staff are taking steps to “resolve the situation permanently,” Blum said.

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