The starkest warning came in mid-September from Bruce Schneier, an internet security expert, who posted a brief essay titled “Someone Is Learning How to Take Down the Internet.” The technique was hardly news: Entities like the North Korean government and extortionists have long used “distributed denial-of-service” attacks to direct a flood of data at sites they do not like.

“If the attacker has a bigger fire hose of data than the defender has,” he wrote, “the attacker wins.”

But in recent times, hackers have been exploring the vulnerabilities of the companies that make up the backbone of the internet — just as states recently saw examinations of the systems that hold their voter registration rolls. Attacks on the companies escalated, Mr. Schneier wrote, “as if the attack were looking for the exact point of failure.” Think of the mighty Maginot Line, tested again and again by the German Army in 1940, until it found the weak point and rolled into Paris.

The difference with the internet is that it is not clear in the United States who is supposed to be protecting it. The network does not belong to the government — or really to anyone. Instead, every organization is responsible for defending its own little piece. Banks, retailers and social media hubs are supposed to invest in protecting their websites, but that does not help much if the connections among them are severed.

The Department of Homeland Security is supposed to provide the baseline of internet defense for the United States, but it is constantly playing catch-up. In recent weeks, it deployed teams to the states to help them find and patch vulnerabilities in their voter registration systems and their networks for reporting results.

The F.B.I. investigates breaches, but that takes time — and, in the meantime, people want to bank online and stream television shows. On Nov. 8, Americans will have to look up where they are supposed to vote, and, in a few cases, they will cast their votes on the internet. Yet the voting system is not considered part of the nation’s “critical infrastructure.”

The head of the National Security Agency, Adm. Michael Rogers, said recently that experts were looking at the problem the wrong way. “We are over-focused on places and things,” he said in a talk at Harvard. “We need to focus on the data,” and how it flows — or doesn’t flow.