What is a domain?

What is Domain, What are domains
What is Domain

What are domains

Each web server is characterized by a unique numeric code, the so-called IP address, which allows a browser to address a specific host to call up the contents of a site. Real users do not see these number sequences for a good reason: IP addresses such as are difficult to remember and it is also very easy to enter them incorrectly. Therefore, another method of redirection was preferred when calling up a site: the domain.

What is a domain?

A domain is a globally unique and distinct name for a specific internet sector, for example a website. Domains appear to users in this form:


As an essential part of a URL (abbreviation for Uniform Resource Locator) the domain indicates where a resource is located within a hierarchically structured DNS (Domain Name System). The translation of domains into IP addresses is done using so-called name servers. These are specialized web servers that are entrusted with the resolution of the name of IP addresses. This service works in a similar way to a normal telephone service: a user enters the domain www.esempio.com in the search mask of his browser and this sends a request to the competent name server. Here, the entry www.esempio.com is called up from the database and the IP address entered is transmitted to the browser.

What is Domain, What are Domain

Structure of a domain

The full name of a domain is called Fully Qualified Domain Name (FQDN). An FDQN indicates the exact location of a target computer in the DNS tree hierarchy and consists of two parts: the computer name (host name) and the domain name. The following example shows the FQDN of a dummy mail server:


While the mailserver reports the name of the host, example.com is the domain name under which the computer in use is located. The host name for the servers, which are responsible for the operation of the sites, is usually the characteristic www:


It should be noted that the Fully Qualified Domain Name, unlike the Internet address, always ends with a dot in common use. This is due to the hierarchical structure of the DNS, according to which domains are always distributed starting from the highest level, the so-called Root-Label.

# Third level domain Second level domain First level domain Root label
1 www example com (empty)

From right to left you will find: root label, top-level domain (TLD), second-level domain (SLD) and third-level domain. The www.esempio.com FQND therefore contains four elements. In principle, domains can have other subdomains under the third-level domain.

Root label

The first layer of the DNS tree root is called the root or null label. The root label of an FQDN is defined as empty and does not usually appear in user applications on the Internet. Name server entries, so-called resource records, on the contrary, must always have the complete FQDN with a dot at the end after the top-level domain: www.example.com.

 What is a domain, What is Subdomain, What are domain

Top Level Domain (TLD)

Since the root domain is defined as empty, top-level domains illustrate the highest level of resolution of the name. TLDs are managed by the so-called Network Information Center (NIC). The tasks of an NIC include the management of name servers and the assignment of second-level domains in the top-level domain. The IANA (Internet Assigned Numbers Authority), a department of ICANN (the American body that manages the Internet network), distinguishes between two groups of top-level domains: generic TLDs, such as .com or .info, and country-specific TLDs, such as .de or .it. Part of the generic TLDs are managed as Sponsored Top Level Domains of special interest groups or companies. The registration of a domain may depend on special requirements or constraints. Since top level domains appear as the last part of a domain, the synonym "domain extension" is quite common. In the example domain, the extension "com" corresponds to the top level domain.

Second level domains (SLD)

The second-level domain is defined as a name freely chosen from the top level domains. As in the case of the name "example" in the space for the name before the .com. SLDs are always assigned in combination with the top-level domain. For the sale to the end user, a private registrar accredited by the competent NIC is usually assigned.

Third level domain

Third level domains are defined as Third Level Domains. In the FQDN in the FQDN they are located to the left of the second-level domain. The third-level domains can be used to define the sub-addresses of a domain, which clearly separate the various domains. Therefore, domain owners have the option of making other destination pages, services or servers available. Common names for the third-level domain are www for web services, m for mobile offers, mail, imap or pop3 for mail servers and various country codes for specific offers in that language. This is achieved with the example of the online encyclopaedia Wikipedia. The offer in several languages is structured via the third-level domain:

# Third level domain Second level domain First level domain Specific path on the server
1 en wikipedia org / Wiki / Main_Page
2 it wikipedia org / Wiki / Main_Page

While the homepage of the English version can be accessed via the third-level domain en, the Italian-language version of the encyclopedia is located under the it extension. In addition, for companies that have several locations and would like to provide a specific regional offer on separate sites in the same second-level domain (for example under the company name), third level domains and possibly other domains under the third level are available.

Third level domains are often referred to as 'subdomains'. Strictly speaking, however, this definition is not limited to third-level domains.

What is a subdomain?

A subdomain is a domain that in the DNS hierarchy is located below another domain. Each domain under a TLD is therefore a subdomain. While an SLD in the structure of a domain constitutes a subdomain of the TLD, the third-level domain is subordinate to the SLD as a subdomain. There are several advantages to this subdomain structure. As the example of the online encyclopaedia Wikipedia shows, the third-level domain is of particular importance. In reality, subdomains below the third level are also possible.

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  1. Thank you for sharing your thoughts and knowledge on this topic. This is really helpful and informative, as this gave me more insight to create more ideas and solutions for my plan. I would love to see more updates from you.

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