Steve Grobman, chief technology officer at Intel Security, compared an outage at a domain name services company to tearing up a map or turning off GPS before driving to the department store. “It doesn’t matter that the store is fully open or operational if you have no idea how to get there,” he said in a telephone interview.

Jason Read, founder of the internet performance monitoring firm CloudHarmony, owned by Gartner Inc., said his company tracked a half-hour-long disruption early Friday in which roughly one in two end users would have found it impossible to access various websites from the East Coast. A second attack later in the day caused disruption to the East and West Coasts as well as impacting some users in Europe.

“It’s been pretty busy for those guys,” Read said. “We’ve been monitoring Dyn for years and this is by far the worst outage event that we’ve observed.”

Read said Dyn provides services to some 6 percent of America’s Fortune 500 companies. That means a lot of disruption.

“It impacted quite a few users,” he said of the morning’s attack.

A full list of affected companies wasn’t immediately available, but major sites including Twitter and coder hangout Github said they briefly experienced problems earlier Friday.

For James Norton, the former deputy secretary at the Department of Homeland Security who now teaches on cybersecurity policy at Johns Hopkins University, the incident was an example of how attacks on key junctures in the network can yield massive disruption.

“I think you can see how fragile the internet network actually is,” he said.

Dyn said in a series of statements that it first became aware of the attack around 7:00 a.m. local time and that services were restored about two hours later. A little more than two hours later, the company said it was working to mitigate another attack. A Dyn spokesman didn’t respond to questions seeking further information about the online onslaught.

The U.S. Department of Homeland Security is monitoring the situation, White House spokesman Josh Earnest told reporters Friday. He said he had no information about who may be behind the disruption.

Security experts have recently expressed concern over increasing power of denial-of-service attacks following high-profile electronic assaults against investigative journalist Brian Krebs and French internet service provider OVH .

In a widely shared essay titled “Someone Is Learning How to Take Down the Internet,” respected security expert Bruce Schneier said last month that major internet infrastructure companies were seeing a series of worrying denial-of-service attacks.

“Someone is extensively testing the core defensive capabilities of the companies that provide critical internet services,” he said.