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Zones and Zone Servers

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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Zones are subsets of DNS (Domain Name System) namespaces or, in the case of the Internet, the DNS namespace. The global DNS namespace includes the names of all sites in an intranet or the Internet, organized hierarchically under the familiar domain "dot com," "dot gov," "dot org," and so on. Within this global DNS namespace are individual zones of authority, which are independently managed domains. The administrative authorities are responsible for managing that part of the domain.

For example, under the .com domain are familiar zones such as or Zones might be further partitioned into subzones. For example, Microsoft has its own zones for products, support, development tools, and so on.

The defining feature of zones is that they contain zone servers, usually two-one primary and one secondary backup system. As for terminology, zone servers = name servers = DNS servers. They are all the same. The primary zone server is where the zone file is managed. The secondary zone server gets a read-only copy of this file.

The zone file is a text file that contains a series of resource records that bind names to values in the following form:

Name, Value, Type, Class, TTL

Name is the DNS name, while Value is an associated IP address. Type defines what the DNS record is and thus what kind of information it contains. Some common record types are as follows:

  • Type = A (Address) Indicates the record is a name to IP address mapping.
  • Type = NS (Name Server) Indicates the record specifies the domain name of servers that provide DNS naming services. A root server in a domain contains an NS record for each second-level server, along with an A record that provides the IP address of the server.
  • Type = CNAME (Canonical Name) The record contains a mapping from one domain to another domain. This record defines aliases.

The Class field defines a way to specify other entities that might define other type of resource records for the zone file. So far, only the Class IN (Internet) has been used. Finally, the TTL (Time To Live) field specifies how long the record from another server should be allowed to remain cached before it is removed.

Clients query zone servers primarily to obtain IP addresses for a given DNS name. The name is submitted to the server and the server returns an IP address. Often, a server may not have information about a particular name, so the request is forwarded up the hierarchy of name servers until the request can be satisfied.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.