Generic top-level domains (gTLDs) are one of the categories of top-level domains (TLDs) maintained by the Internet Assigned Numbers Authority (IANA) for use in the Domain Name System of the Internet. A top-level domain is the last level of every fully qualified domain name. They are called generic for historical reasons; initially, they were contrasted with country-specific TLDs in RFC 920. The core group of generic top-level domains consists of the com, net, org, biz, and info domains. In addition, the domains name, and pro are also considered generic; however, these are designated as restricted, because registrations within them require proof of eligibility within the guidelines set for each. Historically, the group of generic top-level domains included domains, created in the early development of the domain name system, that are now sponsored by designated agencies or organizations and are restricted to specific types of registrants. Thus, domains edu, gov, int, and mil are now considered sponsored top-level domains, much like the themed top-level domains (e.g., jobs). The entire group of domains that do not have a geographic or country designation (see country-code top-level domain) is still often referred to by the term generic TLDs. The number of gTLD as of March 2018 exceeds 1,200 domains. Types Overall, IANA distinguishes the following groups of top-level domains: infrastructure top-level domain (.arpa) generic top-level domains (gTLD) restricted generic top-level domains (grTLD) sponsored top-level domains (sTLD) country code top-level domains (ccTLD) internationalized top-level domains (IDNs) internationalized country code top-level domains (IDN ccTLD) testing top-level domains (tTLD) History The initial set of generic top-level domains, defined by RFC 920 in October 1984, was a set of "general purpose domains": com, edu, gov, mil, org. The net domain was added with the first implementation of these domains. The com, net, and org TLDs, despite their originally specified goals, are now open to use for any purpose. In November 1988, another TLD was introduced, int. This TLD was introduced in response to NATO's request for a domain name which adequately reflected its character as an international organization. It was also originally planned to be used for some Internet infrastructure databases, such as ip6.int, the IPv6 equivalent of in-addr.arpa. However, in May 2000, the Internet Architecture Board proposed to exclude infrastructure databases from the int domain. All new databases of this type would be created in arpa (a legacy domain from the conversion of ARPANET), and existing usage