.io Domain Explained: What is It and Why Do Mobile Games Use It?

You’ve likely noticed the rise of in-browser games using the “.io” domain. Agar.io, Sliter.io, Zombs.io, the list goes on and on. But what the heck do these punchy little URLs mean, and why are so many sites, browser games especially, using the .io TLD?

There are over 1500 possible domain extensions. These short, two letter TLDs are intended to represent website or domain hosted in a specific country. In the age of global internet usage, however, bazillions of websites need to stand out.

Some more prolific region-based websites will use common TLDs for their country, such as .uk or .au. Some of these extensions, generally those representing larger countries actually require proof of residency from the domain’s owner. Most of the smaller ones don’t, like .io.

The history of the .io domain extension

The domain extension .io is intended to represent domains and websites registered or established in the British Indian Ocean Territory. The archipelago of 55 islands nestled in the Indian Ocean. Only one of these islands is actually inhabited, Diego Garcia, with a population of roughly 4200 residents, and almost all of them work in military for the United States and United Kingdom.

As you can probably tell, the .io domain extension does not have any sort of geographically limiting requirements, and it has been generally adopted by technology based-companies. “IO” (or “I/O”) is a common acronym for “input/output,” an obviously common term regularly used in computing and technology. For example, you’ve likely heard of your keyboard being referred to as an “I/O device.”

Because of this, the .io domain is appealing to web developers, technology-based companies, and any other wireheads. Including developers of browser-based games.

Many developers (and certainly users) consider usage of anything other than one of the generic top-level domains (gTLDs) undesirable, more scrutiny is given to domains outside of the conventional, considered spammy. Due to .io's general use in technology, there hasn’t been nearly as much caution regarding .io.

Agar.io, and the birth of a genre

There are certainly many .io domains in use by the aforementioned technology companies, startups, and anyone who happens to want a domain hack using “.io” to form a word (such as http://scenar.io/). However, the most prominent use of “.io” appears as URLs representing a specific genre of browser-based games.

Back in 2015, a young developer by the name of Matheus Valadares, created a free-to-play game in which the player takes the role of a cell (or cells) moving about in a petri dish, with the sole mission of gaining mass by absorbing other cells. The name Agar.io was suggested by a 4chan user. “Agar”referring to a gelatinous substance commonly used for the culturing of bacteria in a petri dish.


Spend money and you can have a cool blob, too!

Agar.io’s gameplay is simple: Use your mouse to float your cell around the screen, eating food and growing larger, with the intention of eating cells smaller than you. You can also split into multiple, smaller cells to reach and absorb other cells, defeating other players. Simple, yet addicting.

Agar.io’s popularity soared since its original debut, likely due to the game’s easy to pick up nature and addictive, Pac-Man-esque “Eat all the things!” goals. Agar.io was soon picked up by YouTube gaming personalities, like PewDiePie, whose videos of the game spread the word. The game received multiple updates and a mobile version, published by the free online gaming site Miniclip.

Send in the clones

Success can often breed imitators. It was certainly the case with Agar.io. Its popularity spawned countless games.

Since many were already copping Agar.io’s gameplay, it made sense to copy the URL. This led to the birth of many similar domains and, ultimately, “.io” became synonymous for not only these Agar.io clones but other browser-based games.


Neat, my mom's in this one twice. Hi mom!

Now the .io TLD is not only for Agar.io clones. One such game is Zombs.io, a tower defense game featuring base-building, with traps and weaponry to protect from zombie invasions. Others expanded on Agar.io’s premise, such as deeeep.io.

Part of the .io popularity are people wanting a piece of that delicious, gelatinous Agar.io pie. But there are other, tasty pies out there. Here’s a few of our favorites: Agar.io, slither.io, Zombs.io, deeeep.io, SkyRoyale.io.

And of course there's the treasure trove of games, itch.io, an alternative to Steam that prioritizes alternative, browser, and indie games. It's also a good site to keep up with various game jams happening around the world.