Domain

Definition:

A domain, or domain name, is a unique name within the Domain Name System hierarchy that identifies Internet resources and services through a human-readable address. arimetrics.com is an example of a domain name.

A domain is not the website or the server that hosts it. It can be used for a website, email addresses and other services. The DNS looks up the records associated with the name and directs each request to the appropriate resource.

How a domain name works

When someone enters a domain name in a browser, a DNS resolver searches a distributed system of servers for the required information. The response may include an IP address or other records indicating where the requested service is located.

The same domain can have different records for different functions. A and AAAA records can direct web traffic to IPv4 or IPv6 addresses, while MX records specify which servers receive email. Registering a domain therefore does not mean that a website already exists: DNS must also be configured and the corresponding service must be available if a site is to be published.

Domain name structure

Domain names have a hierarchical structure. Their labels are separated by dots and are interpreted from right to left. In www.arimetrics.com, the following levels can be distinguished:

Root: This is the highest level of the DNS hierarchy. It is technically represented by a final dot that is normally omitted.
Top-level domain: This is .com in the example and is also known as the TLD.
Second-level domain: This is arimetrics, the label registered under .com.
Subdomain: This is www in the example. It is optional and can be replaced by labels such as blog, shop or mail.

The registrable domain generally consists of the chosen name and the top-level domain, such as arimetrics.com, although registry rules vary. A subdomain can organise services or sections under that domain without requiring a separate registration.

Types of top-level domain

The IANA Root Zone Database classifies TLDs according to their purpose and administration. Its main categories include:

Generic domains: gTLDs include extensions such as .com, .org, .info and .app. Some have eligibility requirements, while others are open to general registration.
Country-code domains: ccTLDs are assigned to countries or territories and usually use two-letter codes, such as .es or .fr.
Sponsored domains: Extensions such as .aero and .museum have a sponsoring organisation and conditions linked to a particular community.
Infrastructure domain: .arpa is reserved for technical Internet infrastructure functions.

Combinations such as gov.uk are not a mixed type of TLD. They are structures created within the .uk country-code domain. The IANA Root Zone Database lists the category and operator of each delegated extension.

Registration and management

To use an available name, a person or organisation registers it for a defined period through a registrar. The registry maintains the TLD database, while the registrar processes registrations, renewals and other changes for the registrant. Rules, terms and eligibility requirements depend on the extension.

ICANN coordinates policies and agreements concerning generic domains. The IANA functions maintain the root zone and TLD delegation data. The registrant should keep contact information current, protect access to the account and renew the domain on time to retain the right to use it.

Related concepts

A domain is one part of web infrastructure and should not be confused with the following concepts:

URL: A URL identifies a particular resource and includes elements such as the scheme, domain, path and, when present, parameters or fragments. In https://www.arimetrics.com/en/digital-glossary/domain, the domain is www.arimetrics.com and /en/digital-glossary/domain is the path.
Hosting: Hosting provides the resources where a website and other services are stored or run. The domain is the name that can direct users to those resources.
Subdomain: This is a label added before the registrable domain to organise services or sections. www is one possible label, not a required component.
DNS: This is the distributed system that stores and answers queries about records associated with domain names.