A service disruption at content hosting company Akamai Technologies cut off access to some of the Internet's major websites Tuesday, including Google.com and Microsoft.com, according to The SANS Institute's Internet Storm Center.

DNS servers that Akamai uses to host services for some customers stopped responding at around 1.30pm, leaving Web surfers who were trying to reach those sites high and dry, said Johannes Ullrich, chief technology officer at the Internet Storm Center. Andy Ellis, Akamai's chief security architect, declined to comment, but did not deny that the company was experiencing problems.

An Akamai spokesman did not respond to requests for comment.

Akamai offers an "Enhanced DNS" service, in which customers use Akamai's network of DNS servers to resolve Internet requests for their domain, such as Google.com. The distributed, worldwide network of servers is marketed by Akamai as a more stable and robust solution than internally managed DNS servers, which are susceptible to failure or attack.

However, users began reporting problems Tuesday reaching popular sites, like Google.com and Yahoo.com, that use the service. The Internet Storm Center received reports of troubles from across the world, including Asia and South America, Ullrich said.

An investigation into the problem revealed that Akamai's DNS servers were not responding to requests. The problem appeared to affect the company's worldwide network of DNS servers, suggesting that the problem may have been caused by a problem within Akamai rather than an external attack on its DNS servers, Ullrich said. "It could be anything - somebody tripping on an power cord. I think an attack is unlikely, unless somebody was able to hit a central control node," he said.

While some Internet users were prevented from reaching the affected sites during the outage, others were unaffected, due to the distributed nature of the DNS infrastructure. DNS, which translates reader-friendly names like "Yahoo.com" into numeric Internet Protocol addresses used on the Internet, is a tree-like structure of distributed servers, each with its own frequently-updated list of server and domain addresses. Problems in one part of the DNS system often do not affect other parts of the infrastructure for hours, or more, Ullrich said.

The hiccup in Akamai's service is the second in less than a month. In May, a software glitch slowed Akamai servers used to host customer Web pages and other information, making it difficult to reach some of its customers' websites for around 90 minutes, Akamai acknowledged.

Akamai blamed that service interruption on a bug in content management software that Akamai customers use to update content on Akamai servers, said Jeff Young, an Akamai spokesman. Yahoo.com, Google.com and Microsoft.com were all reachable again within two hours of the reported interruption.