They are organized into:
1) Subordinate levels (subdomains) of the DNS root domain, that is nameless.
2) First-level set of domain names are the top-level domains (TLDs).
a) Includes the generic top-level domains (gTLDs), such as the prominent domains, com, net, and org.
b) The country code top-level domains (ccTLDs).
3) Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users who wish to connect local area networks to the Internet, run web sites, or create other publicly accessible Internet resources.
The registration of these domain names are administered by the domain name registrars who sell their services to the public. Each Internet host computer use domain names as host identifiers, or hostnames. Hostnames are the leaf labels in the domain name system without further subordinate domain name space. Hostnames appear as a component in Uniform Resource Locators (URLs) for Internet resources such as web sites. Domain names are also used as simple identification labels to indicate ownership or control of resource. Such examples are the realm identifiers used in the Session Initiation Protocol (SIP), the DomainKeys used to verify DNS domains in e-mail systems, and in many other Uniform Resource Identifiers (URIs).
Domain Name System (DNS) is an essential component of functionality of the Internet. A domain name system is a hierarchical naming system for computers, services, or any resource connected to the Internet or a private network. It translates easily identifiable human friendly domain names into the numerical identifiers or IP addresses associated with networking equipment for the purpose of locating and addressing these devices worldwide.
The advantages of DNS is that it makes possible to assign names to groups of internet users in a meaningful way independent of each users physical location and thus world wide web hyperlinks and internet contact information can remain consistent even if the internet routing arrangements change or the participant uses a mobile device. Internet domain names are easier to remember than IP addresses such as 208.77.188.166 (IPv4) or 2001:db8::1f70:6e8 (IPv6) etc. People can memorize meaningful URLs and e-mail addresses without having to know about how the computer locates each of them using IP addresses. The DNS distributes the responsibility of assigning domain names and mapping those names to IP addresses by designating authoritative name servers for each domain. DNS also stores other types of information such as the list of mail servers which accepts email for a given Internet domain and also provides distributed keyword-based redirection service.
Some examples of top level domains:
1) Generic top-level domains or organizational domains.
Domain Description
com Commercial
edu Educational
gov U.S. government
int International
mil Military
net Network providers
org Miscellaneous organizations
2. Country code top-level domains or geographical domains.
Domain Description
at Austria
au Australia
be Belgium
ca Canada
ch Switzerland
cn China
de Germany
dk Denmark
es Spain
fr France
gr Greece
ie Republic of Ireland
in India
it Italy
jp Japan
nz New Zealand
th Thailand
uk United Kingdom
us United States
