Information Technology Reference
In-Depth Information
model is described as being an inverted tree, with the root at the top. The root is represented
by the null set "" . When written, the root node is represented by a single dot ( . ).
Each node in the DNS can branch out to any number of nodes below it. For example,
below the root node are a number of other nodes, commonly referred to as top-level
domains (TLDs) . These are the familiar .com , .net , .org , .gov , .edu , and other such
names. Table 2.1 lists some of these TLDs.
Table 2.1
Common top-level DNS domains
Common top-level
domain names
Type of organization
com
Commercial (for example, stellacon.com for Stellacon Training
Corporation).
edu
Educational (for example, gatech.edu for the Georgia Institute of
Technology)
gov
Government (for example, whitehouse.gov for the White House in
Washington, D.C.)
int
International organizations (for example, nato.int for NATO); this
top-level domain is fairly rare
mil
Military organizations (for example, usmc.mil for the Marine
Corps); there is a separate set of root name servers for this domain
net
Networking organizations and Internet providers (for example,
hiwaay.net for HiWAAY Information Systems); many commercial
organizations have registered names under this domain too
org
Noncommercial organizations (for example, fidonet.org for
FidoNet)
au
Australia
uk
United Kingdom
ca
Canada
us
United States
jp
Japan
Each of these nodes then branches out into another set of domains, and they combine to
form what we refer to as domain names, such as microsoft.com . A domain name identifies
the domain's position in the logical DNS hierarchy in relation to its parent domain by
separating each branch of the tree with a dot. Figure 2.2 shows a few of the top-level
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