|TLD type||Country code top-level domain|
|Registry||NIC.IO (run by Internet Computer Bureau)|
|Sponsor||IO Top Level Domain Registry (Cable and Wireless)|
|Intended use||Entities connected with British Indian Ocean Territory|
|Actual use||Popular with startup companies and browser games; little if anything related to the territory itself.|
|Registration restrictions||None for 2nd level registrations; 3rd level registrant must be resident of British Indian Ocean Territory|
|Structure||Registrations are taken directly at the second level or at third level beneath various 2nd-level labels|
|Documents||Terms & Conditions; Rules|
|Dispute policies||Dispute Resolution Policy|
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.
The right to administer domain names is given to approved organisations by the Internet Assigned Numbers Authority (IANA). The Internet Computer Bureau (ICB) administers .io domains. This domain name registry is a British company, and operates for this purpose under the name NIC.IO. The company also holds the rights to sell the .sh and .ac domains, the top-level domains for the Islands of Saint Helena and Ascension, respectively.
Individuals and organisations are allowed to register .io domains.
Applicants for the registration of .io domains do not need to be registered or established in the British Indian Ocean Territory. Third-level domains, such as "xyz.com.io", can only be registered by an inhabitant of the area. (Since there are no legal, permanent inhabitants of the British Indian Ocean Territory, theoretically no third-level domains will be registered.) Any second-level domains used by NIC.IO and top-level domains cannot be used as a third-level domain. For example, the domains "com.com.io", "org.com.io", and "biz.com.io" are all restricted.
Domain names in .io may not be used, "for any purpose that is sexual or pornographic or that is against the statutory laws of any nation." If this requirement is breached, "NIC.IO reserves the right to immediately deactivate the offending registration."
.io domains may be registered for a minimum of one year, and a maximum of 5 years.
Domain names in .io are priced higher than those in other TLDs. Registering an available .io-domain currently (at 3 September 2020) costs US$90 per annum.
The .io domain has considerable usage unrelated to the British Indian Ocean Territory.
In computer science, "IO" or "I/O" is commonly used as an abbreviation for input/output, which makes the .io domain desirable for services that want to be associated with technology. .io domains are often used for open source projects, application programming interfaces ("APIs"), startup companies, video games, and other online services.
The TLD is also used for domain hacks, as the letters "io" are an ending of many English terms. For example, Rub.io is a shortened URL that was used for the 2016 U.S. presidential campaign of Marco Rubio.
One reason given for the TLD's popularity is that it stands out by being shorter than other TLDs. Also, the .io TLD is less occupied than other TLDs, so it is more likely that a given term is available there.
In Italian, io is the first-person singular pronoun (English "I"), which makes the domain appealing for personal websites.
In Esperanto io as an independent word is the assertive existential indefinite pronoun (English "something"). As a suffix, -io is used to terminate official names of countries or other kind of lands under which a community of people are grouped. The Plena Ilustrita Vortaro de Esperanto include almost 500 terms like that, from Abisenio (former name of Ethiopia, Etiopio) to Zambio (Zambia). Derived from that, the suffix is also used to designate a community of people whose common interest is indicated by the suffixed root, especially in the term Esperantio, the community of speakers of the language and their culture as a whole, as well as the places and institutions where the language is used. As of May 2020, the esperant.io domain name itself redirect to Libera Folio, an independent generalist online bulletin written in Esperanto.
A viral MMO game, Agar.io, spawned many similar MMO games like it that drew from its success, notably Diep.io, paper.io (although paper-io.com is now its home), wormax.io and wormax2.io,Surviv.io, Hole.io, Slither.io, and krunker.io. This trend of using ".io" for these online video games has continued and can be considered an indicator for its category.
According to a Gigaom interview with Paul Kane, chairman of the Internet Computer Bureau, the domain name registry is required to give some of its profits to the British government, for administration of the British Indian Ocean Territory.
After being questioned as a result of the interview, the British Government denied receiving any funds from the sale of .io domain names, and argued that consequently, the profits could not be shared with the Chagossians, the former inhabitants forcibly removed by the British government.
There's lots of open source projects (Redis, Brackets, Launcher), a few mobile-app landing pages (Avocado, X-Ray), a ton of new web apps and services, several conference pages (Lightning, Renaissance, Resonate) and a few older companies or organizations who've changed their name to take advantage of a cleaner .io name.