As reported on twitter and Hacker News, .IO domains have stopped resolving and apparently new registrations are not going through either. .IO is the ccTLD for the British Indian Ocean territory. The TLD is especially popular with startups. According to comments on the post, 5 of 7 nameservers are returning NXDOMAIN (=domain does not exist) for queries for IO domains.

DNN has reached out the registry operators for comment.