BOSTON (AP) — A very strange thing happened on the internet the day President Joe Biden was sworn in. A shadowy company residing at a shared workspace above a Florida bank announced to the world's computer networks that it was now managing a colossal, previously idle chunk of the internet owned by the U.S. Department of Defense.

That real estate has since more than quadrupled to 175 million addresses — about 1/25th the size of the current internet.

"It is massive. That is the biggest thing in the history of the internet," said Doug Madory, director of internet analysis at Kentik, a network ope...