The Internet withstood what appears to have been a major assault on its core infrastructure Tuesday, when all 13 of its root servers were attacked, according to a spokesman for VeriSign, which operates two of the servers.

The distributed denial of service (DDOS) attack started at about 5pm EDT (Eastern District Time) Monday and lasted for about an hour, said Brian O'Shaughnessy, a spokesman at VeriSign.

The US Federal Bureau of Investigation's (FBI's) National Infrastructure Protection Centre "is aware of the matter" and is "addressing" it, said Steven Berry, a supervisory special agent with the FBI's press office.

Root servers are used by the Internet's DNS (domain name system), which takes easy-to-remember domain names used by people – such as – and converts them into the numerical IP addresses used by computers.

Root Four or five of the Internet's 13 root servers kept working during the attack, so Internet traffic kept moving, because the DNS is structured so that eight or more of the servers have to stop working before slowdowns occur, according to a report Tuesday evening in the Washington Post.

In fact, no major outages occurred as a result of the attack, according to the Post, meaning Internet users were unaware of what had happened. Nevertheless, one source quoted in the report characterized the incident as one of the largest attacks ever against the Internet.

"This was the largest and most complex DDOS attack ever against the root server system," an anonymous source at an organization responsible for the system told the Post.