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Top-level domain

Domain at the highest level of the DNS hierarchy

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History

The development of top-level domains followed the introduction of the Domain Name System as a replacement for the earlier flat host-name system, in which host names and addresses were maintained in a central HOSTS.TXT file. The DNS introduced a distributed, hierarchical namespace, with the unnamed root above a set of top-level domains.

Originally, the top-level domain space was organized into three main groups: Countries, Categories, and Multiorganizations. An additional temporary group consisted of only the initial DNS domain, .arpa, and was intended for transitional purposes toward the stabilization of the domain name system. In this early structure, the category domains were .gov, .edu, .com, .mil, and .org, while country domains used two-letter codes derived from ISO 3166. The multiorganization class was reserved for large organizations, especially international organizations, that did not fit easily into the other categories.

The .arpa domain was originally intended for existing ARPA-Internet hosts and was expected to be phased out once hosts moved into other domains. It was later retained and repurposed as an infrastructure top-level domain for technical uses, including reverse DNS mapping.

By 1994, RFC 1591 described the top-level domain structure as consisting of generic top-level domains and two-letter country code top-level domains. The generic domains listed at that time were .com, .edu, .net, .org, .gov, .mil, and .int. The same document described IANA as the overall authority for coordination and management of the DNS, with day-to-day registration responsibilities handled by the InterNIC and regional registries.

The number of generic top-level domains remained small until the Internet Corporation for Assigned Names and Numbers (ICANN) began later expansion rounds. In 2000, ICANN launched an application round that led to the delegation of .aero, .biz, .coop, .info, .museum, .name, and .pro. A further round begun in 2003 led to additional sponsored and generic domains, including .asia, .cat, .jobs, .mobi, .tel, .travel, and .xxx.

A much larger expansion followed ICANN's New gTLD Program, which opened its first application round in 2012. The program allowed applications for many new generic top-level domains, including internationalized domain names in non-Latin scripts, and the first new gTLDs from that round were delegated to the root zone in October 2013.

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