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domains, where the Microsoft domain fits, and a host called Tigger within the
microsoft
.com
domain. If someone wanted to contact that host, they would use the
fully qualified
domain name (FQDN)
,
tigger.microsoft.com
.
An FQDN includes the trailing dot (
.
) to indicate the root node, but it's commonly left
off in practice.
figure 2.2
The DNS hierarchy
(Managed by InterNIC)
Int/Net/Org
Mil
Gov
Edu
Com
ArmyNavy
White
House
IRS
MITStanford
Microsoft
(Managed by
Microsoft)
Tigger
As previously stated, one of the strengths of DNS is the ability to delegate control
over portions of the DNS namespace to multiple organizations. For example, the Internet
Corporation for Assigned Names and Numbers (ICANN) assigns the control over TLDs
to one or more organizations. In turn, those organizations delegate portions of the DNS
namespace to other organizations. For example, when you register a domain name, let's
call it
example.com
, you control the DNS for the portion of the DNS namespace within
example.com
. The registrar controlling the
.com
TLD has delegated control over the
example.com
node in the DNS tree. No other node can be named
example
directly below
the
.com
within the DNS database.
Within the portion of the domain namespace that you control (
example.com
),
you could create host and other records (more on these later). You could also further
subdivide
example.com
and delegate control over those divisions to other organizations
or departments. These divisions are called
subdomains
. For example, you might create
subdomains named for the cities in which the company has branch offices and then delegate
control over those subdomains to the branch offices. The subdomains might be named
losangeles.example.com
,
chicago.example.com
,
portsmouth.example.com
, and so on.
Each domain (or delegated subdomain) is associated with DNS name servers. In other
words, for every node in the DNS, one or more servers can give an authoritative answer to
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