Bind 9 Administrator Reference Manual

Chapter 1. Introduction
For example, a domain name for a host at the company Example, Inc. could be mail.example.com,
where com is the top level domain to which ourhost.example.com belongs, example is a subdomain
of com, and ourhost is the name of the host.
For administrative purposes, the name space is partitioned into areas called zones, each starting at a node
and extending down to the leaf nodes or to nodes where other zones start. The data for each zone is
stored in a name server, which answers queries about the zone using the DNS protocol.
The data associated with each domain name is stored in the form of resource records (RRs). Some of the
supported resource record types are described in Section 6.3.1.
For more detailed information about the design of the DNS and the DNS protocol, please refer to the
standards documents listed in Section A.4.1.
1.4.3. Zones
To properly operate a name server, it is important to understand the difference between a zone and a
domain.
As we stated previously, a zone is a point of delegation in the DNS tree. A zone consists of those
contiguous parts of the domain tree for which a a name server has complete information and over which
it has authority. It contains all domain names from a certain point downward in the domain tree except
those which are delegated to other zones. A delegation point is marked by one or more NS records in the
parent zone, which should be matched by equivalent NS records at the root of the delegated zone.
For instance, consider the example.com domain which includes names such as
host.aaa.example.com and host.bbb.example.com even though the example.com zone includes
only delegations for the aaa.example.com and bbb.example.com zones. A zone can map exactly to
a single domain, but could also include only part of a domain, the rest of which could be delegated to
other name servers. Every name in the DNS tree is a domain, even if it is terminal, that is, has no
subdomains. Every subdomain is a domain and every domain except the root is also a subdomain. The
terminology is not intuitive and we suggest that you read RFCs 1033, 1034 and 1035 to gain a complete
understanding of this difficult and subtle topic.
Though BIND is called a "domain name server", it deals primarily in terms of zones. The master and
slave declarations in the named.conf file specify zones, not domains. When you ask some other site if it
is willing to be a slave server for your domain, you are actually asking for slave service for some
collection of zones.
1.4.4. Authoritative Name Servers
Each zone is served by at least one authoritative name server, which contains the complete data for the
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