Configuring the Domain Name Service

DNS domains and zones

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:

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 your_company.COM.
subdomain: is a domain that branches off another, such as rivers.your_company.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. (See the Name Server Operations Guide for BIND for more information on the SOA resource record.)
propagating information about the domain upwards through the tree through a series of zone transfers.