Domains Basics

Each web server is identified by a unique numeric code. This so-called IP address enables a browser to target specific hosts in order to retrieve web page content. Human Internet users do not usually get to see these sequences of numbers. And for good reason: IP addresses are difficult to remember and are susceptible to typing errors. For this reason, an alphanumeric method of addressing has become established for calling up websites: the domain.

The world’s first registered domain was It was registered on January 1, 1985. The first .com domain was registered on March 15, 1985 by the Symbolics company.

What is a domain?

A domain is a globally unique, unambiguous name for a logically delimited section of the Internet – for example, a website. Internet users encounter domains in this form, for example:

As an essential component of a Uniform Resource Locator (URL), the domain specifies where a resource can be found within the hierarchically structured Domain Name System (DNS). Domains are translated into IP addresses by so-called name servers.

Domain Name System (DNS)

The Domain Name System is like the telephone book of the Internet.

If we call up a website via its domain, the browser must first find out the IP address of the web server on which the domain is located. Similar to searching for a name in a phone book to find out the phone number, the browser searches for the entered domain name in the Domain Name System to find out the corresponding IP address.

So the Domain Name System is a system for converting domain names into IP addresses and vice versa. Without the DNS, we would only be able to access websites via their IP address, which would be quite confusing for us in the long run.

The Domain Name System is not a central database. When a domain is called up, the browser asks the DNS server that is stored in the IP configuration. Usually, this is the router of the Internet access from which the request originates.

Domain structure

The complete name of a domain is called a Fully Qualified Domain Name (FQDN). An FQDN specifies the exact position of a target computer in the tree hierarchy of the Domain Name System and consists of two parts: the computer name (host name) and the domain name. The following example shows the FQDN of a fictitious mail server:

While mailserver represents the host name, indicates the domain under which the specific machine can be found. As a host name for servers responsible for running web pages, the characteristic www is usually used:

Root label

The first level of the DNS root tree 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. Entries on name servers, so-called resource records, on the other hand, must always be present as a complete FQDN with a trailing dot after the top-level domain:

Top-level domain (TLD)

Because the root domain is defined as an empty domain, top-level domains represent the highest level of name resolution. TLDs are administered by so-called Network Information Centers (NICs). NICs are responsible for running name servers and allocating second level domains below a TLD. The IANA (Internet Assigned Numbers Authority), a division of ICANN’s central Internet authority, distinguishes between two main groups of top-level domains: general TLDs such as com or info, and country code TLDs such as us or es.

A subset of generic TLDs are operated as sponsored TLDs by special interest groups or companies. Registration of such a domain may be subject to specific requirements or conditions. Because top-level domains appear as the last link of a domain, a synonym for “domain extension” is generic.

Second Level Domain (SLD)

A second-level domain is defined as a freely selectable name below the top-level domain. For example, an example name in the com namespace. SLDs are always assigned in conjunction with a higher-level domain. A private registrar accredited by a responsible network card is usually trusted with the business of the end user.

Companies often choose their brand name or company name as the domain name. At, “nike” would therefore be the second level domain.

However, it is also common to include keywords in the domain name for which the website is to be optimized later. So-called exact match domains are particularly popular. These are domains that match a user’s search query exactly. For example:

Third-Level Domain

Third-level domains can be used to provide additional domain addresses. Domain owners have the ability to provide other landing pages, services or servers. Common names for third-level domains:

  • www for web (World Wide Web) offerings
  • mail, imap, pop3 for mail servers
  • ftp for data transfer

In addition, the third-level domain is also used to represent offerings in a particular language.