Top Level Domains

The DNS is a hierarchical structure of names, at the top of the pyramid there is the root zone, which is hosted by the root servers and is managed by IANA (Internet Assigned Numbers Authority).

The root zone is where the Top Level Domains (TLD) are created. Initially in the RFC 920 six TLDs were defined, each of them intended to hold a different kind of institution, these were:

  • arpa (Address and Routing Parameter Area): uses related with the Internet core infrastructure.
  • gov: government related institutions.
  • edu: educational institutions (mostly universities).
  • com: companies and commercial uses.
  • mil: military institutions.
  • org: other organizations (institutions not included in any of the above).

Additionally the RFC 920 approved the creation of country TLDs following the two letter country codes of the ISO maintained standard ISO-3166.

Afterwards two new domains were added: net which was for uses related with the Internet and int which was created at the request of NATO for international institutions, NATO wanted a TLD that reflected its international nature, the int TLD is managed directly by IANA.

Initially the creation of TLDs was managed by IANA but at the end of 1998 a new institution called ICANN (Internet Corporation for Assigned Names and Numbers) was created with the mandate of managing the TLDs structure, leaving to IANA only the technical management of infrastructure. Under the ICANN management new TLDs were created and the structure was reorganized so there are four kind of TLDs.

Most classifications fit this one into the generic category but it's special enough to deserve it's own category. It only has a single TLD, arpa, and as it's mentioned above it's only used for the management of critical infrastructure and is managed by IANA. The main second level domain below arpa is in-addr which is used for reverse IPv4 resolution: obtaining the domain name matching an IP address.
Generics (gTLD):
Are those TLDs that are open to any person or organization, these are: biz, com, info, name, net, org and pro.
Sponsored (sTLD):
Actually a subclass of generics and quite often refered as sponsored generics and included in the gTLD group (ICANN usually considers gTLDs all TLDs except country code TLD). sTLDs are the TLDs that are managed by institutions that control the access to them based on fitting in a certain profile or objectives defined in their mandate, which was approved by ICANN when the TLD was approved. These TLDs are: aero, asia, cat, coop, edu, gov, int, jobs, mil, mobi, museum, tel and travel.
Country Code (ccTLD):
ccTLDs are TLDs that represent countries, and with the exception of Great Britain that uses uk instead of gb they use the country codes defined in the ISO-3166 standard.

The creation of new ccTLDs is regulated by the RFC 1591, that textually cites:

The IANA is not in the business of deciding what is and what is
not a country.

The selection of the ISO 3166 list as a basis for country code
top-level domain names was made with the knowledge that ISO has a
procedure for determining which entities should be and should not
be on that list.

This essentially means that IANA, that is in charge of adding and removing new TLDs in the root servers, uses the ISO-3166 as the base to decide the validity of a new request for a ccTLD, once a country disappears from that standard a transition process is started that ends in the removal of the ccTLD (like for example cs which was initially assigned to Czechoslovakia, then to Serbia and Montenegro, and currently is unused). At this time there three domains in process of being phased out: su (Soviet Union), tp (Portuguese Timor) and yu (Yugoslavia)

There are some restrictions about what text strings may constitute a TLD:

  • The RFC 2606 reserves several TLDs for using in documentation and testing, these are: test, example, invalid and localhost. Additionally some second level domains are reserved:, and
  • ICANN does not accept requests for gTLDs and sTLDs that are one or two letter strings, the latter being reserved for ccTLDs and, contrary to popular believe the domains fm and tv have no relation to radio or television, they belong instead to the Federated States of Micronesia and to Tuvalu respectively.

IANA publishes the list of all TLDs in the root zone as well as some information about the entities responsible of their management:

At this time the ICANN has two open processes for TLD reform:

  • New gTLD creation process named New gTLD Program, in which the existing procedures are reviewed with the objective of relaxing the process and open it to more organizations and institutions.
  • Internationalized Domain Names (IDN), defined in RFCs 3490, 3491 and 3492 allows the implementation of internationalized domain names using UTF-8. At this time ICANN is working on the introduction of IDN TLDs (specially IDN ccTLDs but also generics).

At this point we can understand the Top Level Domain (TLD) structure, but these domains are not directly used by institutions, companies and persons to identify their domains, they just form the first level of delegation from the root and are grouped by groups of interest, countries or just simply generics. The final users (companies, institutions or individual people) generally use second level domains, except a few special cases where the ccTLD managers create a few second level domains and what is commercialized are third level domains, an example is uk in which the creation of second level domains is strongly restricted (,,, etc...) and third level domains are commercialized instead.

In order to understand the second level domain creation process some new concepts have to be introduced:

  • Registry: is the authoritative repository containing all the information refering a TLD, a full list of all TLDs and their Registries can be obtained from IANA's web site. These are unique for each TLD.
  • Registrant: is the final client, the person or institution who wants to obtain a second level domain. Even though is not frequent it is possible for a Registrant to obtain a delegation of its zone and manage it personally.
  • Registrar: is the entity that intermediates between the Registrant and the Registry, generally it offers the option of managing the delegated zone and some additional services (mail management, web space, etc...). They act as distributors of the TLDs and generally there are several Registrars for each TLD.

This way the process of obtaining a new second level domain is done through a Registrar, who takes charge of setting up the new domain in the authoritative registry for the selected TLD, and the domain may be managed either by the registrar or through a delegation by the end user.