The core servers that direct email and Web surfers to their desired destinations around the world have been attacked in an apparently coordinated attempt to cripple the Internet.
Authorities say the attack, which largely failed, was launched at about 7:00am (AEST) yesterday.
Paul Vixie, the chairman of the California-based Internet Software Consortium, which operates one of the root servers, says the attack targeted the 13 root servers that make up Internet's Domain Name System.
He says the attack lasted about an hour.
Steven Berry, a supervisory special agent at the FBI, says the bureau's National Infrastructure Protection Centre was "aware of the issue and we are addressing it".
He declined to comment further.
Experts say the so-called "distributed denial of service" attack congested some traffic but would not have been noticeable to average Internet users.
Mr Vixie said: "It was like redirecting all traffic between Highway 101 and the street you live on, or into your driveway.
"You would not be able to get home because the street in front of your house would be full of cars from [Highway 101]."
Denial of service attacks are designed to temporarily shut down servers by overwhelming them with too much traffic, usually coming from drone computers around the Internet.
The Domain Name System - which matches up the long numerical codes computers use to match other computers attached to the Internet with the Web addresses people type in - and the root servers it relies on for address information, have long been considered the Achilles heel of the Internet, capable of shutting down the network if attacked.
Mr Vixie says the attack on the servers proves the Internet will not be so easily toppled, adding that the Internet is designed to route around obstructions.
"What we learned yesterday is ... it is hard to kill this system," Mr Vixie said. "The Internet is sort of the cockroach of the modern age. It survives.
"We've known all along that this could happen and it does happen periodically against root servers," he added. "It was interesting because it was an attack on all 13 root servers. That's kind of rare."
Peter Salus, the chief knowledge officer at Texas-based Matrix NetSystems, which monitors Internet performance, has speculated that the root server attacks were related to a distributed denial of service attack on a number of Web sites that lasted a few hours later in the day.
"My guess is script kiddies having a good time earlier decided on a new target," Mr Salus said.
He says that of the 13 root servers, those that were the worst affected were the ones operated by: the US Department of Defence Information Centre in Vienna, Virginia; the US Army Research Lab in Aberdeen, Maryland; the Internet Corporation for Assigned Names and Numbers in Los Angeles; and one each in Stockholm and Tokyo.
Ted Julian, co-founder of computer security company Arbor Networks, says the root server attack "is just another reminder that distributed denial of service attacks remain, arguably, the number one threat we face" on the Internet.