The domain basics

Every web server is identified through a unique numerical code known as an IP address. This address enables browsers to speak to specific hosts in order to access website content. Internet users don’t usually ever see these number sequences and don’t really need to either: IP addresses like 93.184.216.34 aren’t particularly easy to remember and can be prone to typos as well. When logging on to a website, an alphanumeric method is used for the address: the domain.

A domain is a worldwide unique and unambiguous name for a logically defined section of the internet; a website, for example. Domains appear in this form:

www.example.com

A domain is an essential part of a uniform resource Llocator (URL) and indicates where a web resource can be found within a hierarchically structured domain name system (DNS). The name server is in charge of translating the domain into an IP address. Specialized web servers are then entrusted with resolving IP addresses. This process works in a similar way to a directory assistance service: a user writes the domain www.example.com into the browser’s search bar and a request is then sent to the corresponding name server . Upon arrival www.example.com is then retrieved from the database and the deposited IP address is transmitted to the browser.

A complete domain name is known as a fully qualified domain name (FQDN). A FQDN states the exact position of a target computer within the tree hierarchy of the domain name system and is made up of two parts: the host name and the domain name. The following example is a fictitious mail server’s FQDN

mailserver.example.com.

While  mailserver represents the host name, example.com indicates the domain where the specific computer can be found. For server host names responsible for operating websites, the typical www is used:

www.example.com.

Note that the fully qualified domain name is different to internet addresses in everyday use in that they end with a period. This is due to the hierarchical structure of the domain name system and as a result, domains are itemized starting from the highest level and ending with the root label.

From right to left, the domain example structure is as follows: root label, top-level domain (TLD), second-level domain (SLD), and third-level domain. The FQND from www.example.com. even contains 4 sections. In principle, domains can contain further subdomains below the third-level domain.

Top-level domain (TLD)

Since the root domain is defined as empty, top-level domains represent the highest level of name resolution. TLDs are managed by network information centers (NIC). The tasks that an NIC has to perform include operating a name server and allocating second-level domains underneath the TLD. The IANA (Internet Assigned Numbers Authority), a department of the central internet management ICANN, differentiates between two main groups of top-level domains:

Generic TLDs like .com and .net, and country-specific TLDs like .us or .co.uk. Some of the generic TLDs are operated as sponsored top-level domains from special interest groups or businesses. Registering such a domain can be associated with special requirements or conditions. Since top-level domains appear as the last part of a domain, they are often referred to as ‘domain endings’. Regarding the example domain, the ending .com corresponds to the top-level domain.

Second-level domain (SLD)

A second-level domain is a freely chosen name below a top-level domain. An example is example in the namespace com. Allocating SLDs  is always done in tandem with the superior TLD. An NIC-accredited private sector register is normally responsible for name allocation for business to customer purposes.

Third- level-domain

Domains on the third level are known, of course, as third-level domains. In an FQDN these are placed in front of the second-level domain. By including a third-level domain, you can define a sub-address, which separates different sections from each other. Domain owners have the possibility to provide other landing pages, services, or servers. Common names for third-level domains are www for web services, m for mobile services, mail, imap, or pop3 for mail servers as well as various country codes for language-specific services. A clear example is the online encyclopedia, Wikipedia. The site can be offered in different languages thanks to the third-level domain:

The English language homepage can be accessed when the third-level domain en is used. By using de you will be directed to the homepage in the German language. A third-level domain also offers companies the chance to show that they have branches in different locations and want to offer a regionally-targeted service or product on a separate website under the same second-level domain (e.g. the company name).

Third-level domains are often referred to as ‘subdomains’. This term isn’t just limited to the third-level domains, though.

A subdomain is a domain that is located below another domain in the DNS hierarchy, and so every domain below a TLD is a subdomain. While the SLD in the domain structure represents a TLD’s subdomain, the SLD’s third-level domain takes on the role of a subdomain. One advantage of a subdomain is that they can be used to classify a domain in a logical way. As shown by the Wikipedia example, the third-level domain has a special significance. Subdomains below the third level are also possible.