Domain Name Service  «Prev  Next»
Lesson 3 Top-level domains
Objective Define top-level subdomains of the DNS

Top-level Domains

Under the current organization of the Domain Name Service (which may change soon), the root domain is subdivided into a set of top-level subdomains. These subdomains are intended to contain all hosts belonging to organizations of particular types. They are described in the table below.
Top-level subdomains, more commonly known as second-level domains (SLDs), are a key component of the Domain Name System (DNS) hierarchy. The DNS is a distributed and hierarchical system that translates human-readable domain names into IP addresses, enabling users to access resources on the internet.
Top-level subdomains are the direct subdivisions of the top-level domain (TLD) in the DNS hierarchy. They are positioned immediately to the left of the TLD in a domain name, separated by a period (.). For example, in the domain name '', 'example' is the second-level domain or top-level subdomain, and '.com' is the top-level domain.
There are several types of second-level domains, which can be grouped into the following categories:
  1. Generic Second-Level Domains: These are the most common types of top-level subdomains and are often used to represent the purpose or nature of a website. Examples include '', '', or ''.
  2. Country Code Second-Level Domains (ccSLDs): These SLDs are based on the two-letter country code (ISO 3166-1 alpha-2) of a particular country or territory. They are typically used to denote websites that are specific to a certain region or serve a localized audience. For example, '' is a second-level domain for a website in the United Kingdom, while '' is a second-level domain for a website in Australia.
  3. Internationalized Domain Names (IDNs): IDNs are second-level domains that use non-Latin scripts or characters, such as Arabic, Chinese, or Cyrillic. IDNs allow domain names to be represented in local languages, making the internet more accessible to a diverse global audience. For example, '例子.中国' is an IDN SLD for a website in China.
  4. Brand Second-Level Domains: Some organizations and businesses register their brand name as a second-level domain to reinforce their brand identity and provide a memorable web address for their customers. Examples include '' and ''.

Top-level subdomains, also known as second-level domains, are essential components of the Domain Name System hierarchy. They represent the subdivisions of the top-level domain and serve various purposes, including denoting the nature of a website, its regional focus, its local language, or its association with a particular brand.

Top-level Domain Description
arpa ARPAnet domain (now obsolete)
com Commercial organizations
edu Educational organizations
gov Civilian government organizations
mil Military organizations
net Network support centers
org Other organizations (non-profits, lobbying groups, Political Action Committees)
int International organizations
country code Geographic code for each country;
for example:
us=United States
uk=United Kingdom

Below these top-level domain names, the managers of the DNS delegate further subdivision of the DNS namespace to organizations with networks connected to the Internet.
This delegation takes place through the process of domain name registration, in which an organization registers its chosen name and associated network addresses with the InterNIC and its agent, the private company Network Solutions, Inc.
Once an organization registers its domain name (such as or, it is free to further subdivide that name. Thus the owners of the domain name may then freely create,, and so forth.


All name servers read the identities of the root servers from a local config file or have them built into the code. The root servers know the name servers for com, net, edu, fi, de, and other top-level domains. Farther down the chain, edu knows about,, and so on. Each domain can delegate authority for its subdomains to other servers. Let us inspect a real example. Suppose we want to look up the address for the machine from the machine The host lair asks its local name server,, to figure out the answer.