Under DNS, the network consists of a hierarchy of domains.
This hierarchy, referred to as the domain name space, is organized
as an inverted tree radiating from a single root, much like the UNIX
filesystem:
Figure 6-1 Domain name space
root domain
is the root of the tree. Final attempts at resolution occur here, if
lower-level servers do not have the requested data.
nodes
are hosts located where the tree branches, or hosts at the end of a branch:
individual computers running DNS software.
domain
is a subtree of the domain name space, sharing its domain name
with the name of its top-most node, such as mynet.COM.
subdomain
is a domain that branches off another, such as rivers.mynet.COM.
zone
is a portion of the domain name space that is served by a primary
name server and one or more secondary name servers. A
zone may be an entire domain, a domain with all of its child domains,
or a portion of a domain.
The top level of the domain name space is fairly well defined, with
domains such as COM for corporations, GOV for
governmental organizations, and EDU for educational institutions.
Administrators build branches of the tree at individual sites by:
registering
a domain name
with the appropriate body.
installing one or more machines as DNS servers for the domain.
configuring these servers' DNS database files to specify
the boundaries of the domain, via the
SOA (start of authority)
resource record.
propagating information about the domain upwards through the tree through
a series of zone transfers.