The DNS name space is structured hierarchically in the form of a tree and divided into different domain levels. It has a root called the root domain, which serves as the anchor for all search transactions within the DNS name space, and several subtrees, which may form independent administration units as so-called zones.
Structure of the DNS name space
The figure 1 illustrates the domain structure of the DNS name space.
| " " | The root domain |
  | |
gov arpa de fr nl others | Top-level domains |
  | |
others xyz univers mycompany | Second-level domains |
 | |
| venus earth mars | Local administration domains |
 | |
others telstar moon eurostar | Individual computers and subdomains |
| | |
| star1 star2 star3 |
Figure 1: Domain structure of the DNS name space
There are DNS servers at each level of the DNS name space. A DNS server is a computer that performs the following tasks:
managing information on servers at the next lower name level, and
mapping of symbolic names to addresses in cases where no subordinate DNS servers for those names exist
The domains of the DNS name space have the following features:
Root domain
The root domain is at the very top of the DNS hierarchy and is managed by ICANN. In the root domain, ICANN maintains root domain DNS servers which, in turn, manage the information on the DNS servers of the next lower level.
Top-level domains
The top-level domains are in the level below the root domain.
These are the top-level domains in the United States:
aero
biz
com
coop
edu
gov
info
int
mil
museum
name
net
org
pro
Air-transport industry
Businesses
Commercial organizations
Cooperatives
Educational institutions
American government institutions
Unrestricted use
International organizations
American military institutions
Museums
For registration by individuals
Network organizations
Non-commercial organizations
Accountants, lawyers, physicians, and other professionals
The top-level domains outside the USA are organized on a country basis. The ISO country codes are used as the names of the countries concerned, e.g. DE for Germany, FR for France, etc. When a new network is registered, the NIC assigns it to the appropriate domain.
Second-level domains
Each of the top-level domains branches into several second-level domains. The organizations based on this level nominate domain administrators who are responsible for managing the name servers of their networks. The NIC additionally nominates a central technical office to manage and coordinate general administration issues across domains.
Local administration domains
The local administration domains are on the level below the second-level domains of the DNS hierarchy. These domains are administered independently by network providers at their own responsibility. The sizes of local administration domains differ greatly. There are some which consist of only one computer, while others include several computers and even additional DNS servers. A local domain can also have further administration domains (so-called subdomains) subordinate to it.
The in-addr.arpa domain is a special domain that is used for converting IP addresses to DNS names. IP addresses are entered in the in-addr.arpa domain in reversed decimal dotted notation.
The DNS concept places no restrictions on converting DNS names to IP addresses, and vice versa. It is therefore also possible to assign multiple DNS names to one IP address or multiple IP addresses to one DNS name.
Zones
A zone defines a part of the DNS name space that is administered by a master server (primary name server). A zone is not restricted to an administration domain and may also include some or all subordinate domains.
Zone data file are files from where the name servers load their zone data.