This is truly — as Marc Rotenberg, executive director of the Electronic Privacy Information Center put it — one of those “I told you so” moments.

For every video on YouTube, the judge required Google to turn over to Viacom the login name of every user who had watched it, and the address of their computer, known as an I.P. or Internet protocol address.

Both companies have argued that I.P. addresses alone cannot be used to unmask the identities of individuals with certainty. But in many cases, technology experts and others have been able to link I.P. addresses to individuals using other records of their online activities.

The amount of data covered by the order is staggering, as it includes every video watched on YouTube since its founding in 2005. In April alone, 82 million people in the United States watched 4.1 billion clips there, according to comScore. Some experts say virtually every Internet user has visited YouTube.

Of course Viacom swears blind that the only people who will have access to this information are its lawyers (who are working on its $1 billion copyright infringement suit against Google). But it brings one up sharply against the implications of cloud computing.