The different types of domains - what are they?

A domain forms an important part of a Uniform Resource Locator (URL) - in everyday life and colloquial language, it is usually simply referred to as a web address. It is a subset of the internet that is distinguished from other segments by a unique identifier consisting of two or more letters. To enable a specific assignment to a segment, there are several types of domains that result from partly content-related and partly technical specifications. Different organisations are responsible for managing the various types of domains, which can serve governmental, private sector, commercial, or non-profit purposes.

What is the structure of a URL and a domain?

A domain fundamentally uses a hierarchical structure and consists of at least two different components that, when combined, create a unique and unmistakable combination. On a technical level, different types of domains can be distinguished:

• Top Level Domain (TLD): highest level in the resolution of names
• Second Level Domain: a largely freely chosen and officially registered address
• Third Level Domain: another subdomain that the owner of a Second Level Domain can set up
• Fourth Level Domain and higher: further subdivisions under the Third Level Domain
• Subdomain: another term for a Third Level Domain or higher

Theoretically, an unlimited number of subdomains can be created to, for example, achieve differentiation for various purposes, customers, or content. However, there is a limitation that arises from the technical nature of a URL. This forms an absolute and unique address, also known as a Fully Qualified Domain Name (FQDN). Being a 16-bit encoding, the name can contain a maximum of 253 characters, with a . as a separator between each individual subdomain. Therefore, in the current system, a maximum of 126 different levels are possible when short domain types with two letters like a country code serve as TLDs.

What are different types of domains?

The modern internet emerged in the early 1990s from a decentralised network of globally distributed mainframe computers. Originally, it served a military purpose of enabling information exchange even in the event of failures in central nodes or infrastructure sectors. Early on, the scientific community also utilised these capabilities to exchange results nationally and internationally. The introduction of the first types of domains took place in the mid-1980s, replacing the abstract IPv4 addresses, which consisted of a four-digit numerical code. The initial types of domains exclusively included TLDs, with each Top Level Domain being assigned a specific purpose. Some of the most important ones are:

• .mil: Military for military facilities and organisations
• .com: Commercial for businesses and commercial purposes
• .org: Organisation for non-commercial or non-governmental organisations
• .net: Network for technical infrastructure and nodes
• .info: Information for news and other informative purposes
• .gov: Government for US government institutions
• .edu: Education for educational institutions including schools, universities, and research institutes

Just a few years after the introduction of the Top Level Domain for computer addressing, it became evident that the number of available names for a Second Level Domain did not meet the practical demand. Consequently, new types of domains were gradually introduced to provide each interested party with their own suitable Second Level Domain.

What types of domains are there?

There are various ways to categorise domain types. A common classification is based on their origin and the organisations responsible for their management. In this system, the following domain types exist:

• Generic Top Level Domain, which were the first types of domains and are divided into sponsored and unsponsored domains
• Unsponsored Domains (uTLD), used by specific groups or intended for use, such as .com, .net, or .info
• Sponsored Domains (sTLD), proposed by specific companies or organizations that take over management
• Country Code Domains (ccTLD), representing a country like .de, .at, or .uk, managed by the official government of that country
• New Domains (nTLD), introduced from the summer of 2008 to create more addresses for universal or specific purposes

The introduction and management of a Top Level Domain is decided by Internet Corporation for Assigned Names and Numbers (ICANN) based in the USA, created in 1998 for this purpose as a non-governmental and non-profit organisation. It delegates the responsibility for existing and new domain extensions to independent organisations that must provide and maintain the necessary infrastructure. An exception is made for ccTLDs, where the allocation and management are directly the responsibility of the officially recognised government of the respective countries. In most cases, scientific or technical institutions take on this task, specifically established for this purpose, and in exceptional cases - such as with the .to Domain, .tv Domain, or .co Domain - also serve commercial purposes. These TLDs are also interesting as ambiguous domain extensions for website operators or for domain trading.

Who is responsible for a subdomain?

Following the hierarchical principle, any holder of a Second Level Domain can set up any Third Level Domain according to their own interests, either for personal use or to provide to external parties. It is a common practice among administrators to use a Third Level Domain with a name like mail.example.org, responsible for email communication. Other commonly used variations for a subdomain include:

• FTP for file transfer using the FTP protocol
• Name or DNS for custom nameservers for address resolution
• VPN as an externally accessible access point via a VPN network for the local intranet
• M for content optimised for mobile devices
• A country or language code like de, fr, or es for internationally active companies with multilingual websites
• A name or randomly generated code for customers' websites

By far, the most commonly used subdomain is undoubtedly the Third Level Domain www, which originates from the early days of the internet. This abbreviation for World Wide Web was almost obligatory for websites in the 1990s, distinguishing them from other domain types like FTP or mail. Nowadays, it is considered outdated and obsolete, but is still frequently used for reasons of compatibility and accessibility to redirect to the original website. Another special case is addresses like .co.uk, where the administrator explicitly dictates the use of the Second Level Domain, leading the Third Level Domain to take on its role.

Essentially, a Third Level Domain and any additional subdomains are subject to the same requirements as a Second Level Domain. This means they can be independently managed like the latter and assigned to a web hosting space, a Virtual Private Server (VPS), or a Dedicated Server. Some web hosts utilise this to provide their customers with a chosen or predefined Third Level Domain. In some cases, customers can use it like their own Second Level Domain and set up nameservers for it or configure additional levels like a Fourth Level Domain.

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