.io games

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.io games or IO games are a genre of free, online multiplayer browser games that gained popularity with the success of Agar.io in 2015.^[1] The games are usually characterized by simple graphics and gameplay in a free for all multiplayer arena. The term ".io" comes from the .io domain, which was originally assigned to the British Indian Ocean Territory^[2] but became popular for game developers due to its short and memorable nature.

Thousands of .io games exist, and many of them have been subject to controversy for both political use and lack of moderation.^[3]

The .io domain was delegated by the Internet Assigned Numbers Authority to British entrepreneur Paul Kane in 1997 together with the ccTLDs .ac (Ascension Island), .sh (St Helena), and .tm (Turkmenistan).^[4] The first subdomain was registered under .io in 1998, when Levi Strauss & Co. registered the domain levi.io.^[5]

Labels for .io domains may only contain alphanumeric characters and hyphens, and must be between 3 and 63 characters long. Domain names cannot begin or end with a hyphen symbol, and may not contain two consecutive hyphens. The entire domain name may not contain more than 253 characters.^[6]

Agar.io is considered the first ever .io game. The game was announced on 4chan on 27 April, 2015 by Matheus Valadares,^[7] a then 19-year-old Brazilian developer. The game, which has mechanics similar to Osmos, another less popular game.layers control one or more circular cells in a map representing a Petri dish. The goal is to gain as much mass as possible by eating cells and player cells smaller than the player's cell while avoiding larger ones which can eat the player's cells.

Unlinke Osmos, Agar.io went viral, and was featured on the free online games site Miniclip.^[3] The mobile version of Agar.io for iOS was released on 8 July 2015 and Android on 7 July 2015 by Miniclip. The browser version was released in June 2015 by CrazyGames.^[8] The Youtuber PewDiePie created a video titled "REDDIT WANTS TO EAT ME! (Agario Part 1) | PewDiePie," which was over 8.5 million views as of July 2024.^[9] The creation and quickly growing popularity of Agar.io^[10][11] led to the creation of similar games using the .io domain, and eventually the .io game genre.^[1]

Slither.io was the second .io game to be released, which is a free for all multiplayer game that is in the Snake genre. The basic premise of the game has 50 players compete to eat colored orbs and grow as large as possible, while destroying other player's snakes.^[12] The game was created in 2016 by Steven Howse, a self-taught independent developer who was inspired to make it after playing Agar.io. The game quickly rose to be the top game on many platforms.^[13]

Starting in around 2016, soon after the popularity spikes of Agar.io and Slither.io, more games in the .io games genre began to be released. Many of these games were simple clones of popular games, usually released in a top down-format.^[1] Some notable games released in this period include Diep.io, another game by Matheus Valadares,^[14] ZombsRoyale.io,^[15] Krunker.io, Wings.io, Surviv.io, and Hole.io. These games all remain popular and are some of the most played games in the .io games genre.

During Covid-19, .io games became very popular because of their accessibility on the web. Most games, not just .io games, however, also experienced growth during this time.^[16]

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