As to Mr. Giuliani’s argument that the second “period no space” instance in his original tweet didn’t create a similar link, that’s because Helsinki.Either, so far, is not a working domain.

Many Twitter users couldn’t resist noting that Mr. Giuliani, who spent 16 years as a security consultant, was originally brought into the Trump administration as a cybersecurity adviser. (He certainly has more digital know-how than Japan’s cybersecurity chief, who recently admitted that he doesn’t use computers.)

It’s not uncommon for political figures to display a lack of understanding regarding the machinery of the internet. Ted Stevens, a former senator from Alaska, was roundly mocked in 2006 when he described the internet as “a series of tubes.”

[Sign up for the Morning Briefing newsletter for a look at what you need to know to begin your day.]

Nor is it the first time politicians have run into issues related to domain names. Senator Ted Cruz of Texas, for example, doesn’t own the www.TedCruz.com domain, which led to the site displaying a pro-Hillary Clinton message shortly after he dropped out of the presidential race in 2016.

Mr. Velazquez said he’d had little time to carry out his mischief last week. He saw Mr. Giuliani’s tweet at 6:20 p.m. and had to act in an improv show at 7 p.m.

He was at another improv show on Tuesday when he saw Mr. Giuliani’s follow-up. He said he was bewildered.

“He could have deleted it and everyone would have forgotten about it, my tweet would have stopped going viral,” he said. “Instead he tweeted about it and created a conspiracy theory against Twitter.”