When Thomas Erdbrink, The Washington Post’s correspondent in Tehran, logs on to the Internet in Iran, he never knows whether Gmail and Google Reader, The Post or Facebook will open for him. Increasingly, this is the error message he sees instead of the page he was trying to reach:

Translation: “According to computer crime regulations, access to this Web site is denied.”

Iranian bloggers have long used a workaround, by connecting to the Internet and then switching to a special connection that bypasses the country’s extremely effective firewall. But Erdbrink reports Thursday that the software recently has stopped working.

Many fear the failing software indicates Iran’s so-called “National Internet” is on the horizon. The government has said the National Internet can be launched at any time. Erdrink reports:

The government’s technology officials have announced the construction of a domestic internet network comparable to an office intranet, which would block many popular sites .... Officials stress that there will still be access to the Web — just not to the “damaging” sites. But Iranian internet users and activists fear that the activation of the National Internet will cut them off from the rest of the world, and put them under increased surveillance by authorities.

“Basically they are already shutting off access to all interesting Web sites,” prominent Iranian blogger Maysam — who spoke on the condition that the last name not to be used out of fear of being summoned by Iran’s cyber police, — told Erdbrink. “We will resemble an isolated island in a changing world if this happens.”

Erdbrink has already felt that isolation:

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