Internet advertising company DoubleClick was shut down yesterday by a denial of service (DOS) attack, a company spokeswoman confirmed.

The attack began at around 10.30am ET (3.30pm BST) in the US and crippled the company's Web site and its advertising servers, which distribute Web advertisements to other 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 Washington Post, New York Times, Cnet, Nortel, and Infoworld Magazine, according to Keynote Systems, a Web performance measurement company.

Denial of ads service

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 yesterday morning. DoubleClick's servers had returned to normal by late afternoon.

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