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A DNS zone is a portion of the global DNS namespace. This namespace is defined by RFC 1034, "Domain Names - Concepts and Facilities" and RFC 1035, ""Domain Names - Implementation and Specification", and is laid out in a tree structure from right to left, such that divisions of the namespace are performed by prepending a series of characters followed by period ('.'), to the upper namespace (e.g. 'com.' to 'wikipedia.com.'). The correlation between domain names and DNS zones is that each period indicates a break in DNS namespace where authority can be delegated. This delegation may be administrative (as in top-level domains which are tracked by organizations and governments) or technical (where a system becomes responsible for the management of information within that zone) or both. Thus a reasonable definition of a zone would be a portion of the DNS namespace where responsibility has been delegated. In the context of the current global domain system, root domain is administered by a set of 13 root servers, and delegation is handed down to organizations that administer the top-level domains (TLDs) like 'com.', 'net.', 'org.' as well as country domains like 'ie.'. Thus on a global level the DNS root may be considered a zone, as may each of the TLDs. The majority (probably all) of these high-level delegations are both administrative and technical, but specific delegation of zones beneath these defined root elements is dependent on the organizations involved. For example, the 'uk.' domain is further broken into specific zones for specific purposes (e.g. 'co.uk.' is for companies) before being delegated to individual people or organizations. Once we reach a locally (i.e. single organization) administered portion of the DNS namespace, it becomes more flexible. However, a domain name will be represented by a single zone in the majority of cases. That zone will manage the entire domain's namespace serving out information like Internet addresses (i.e. translating "en.wikipedia.org." to 145.97.39.155) even when more complex logical divisions of the namespace are used (for example, we create a subnet and name it internal.wikipedia.org. and prepend desktop system names to it). In more complicated scenarios a zone (e.g. 'wikipedia.org.') may well be delegated, perhaps numerous times, so that there will be a distinct 'internal.mycompany.com' zone and an 'external.mycompany.com' zone. These zones may then be administered, updated, and managed on entirely distinct systems.
Common DNS zones
The most common DNS zones are the ones already mentioned, the ubiquitous DNS root zone, and the TLDs. These are the domains that people commonly purchase to represent their companies or personal presence on the Internet (or more precisely, they purchase it to obtain a website address), but there are other zones within DNS system that play a vital role in the Internet. The most important of these domains is the 'in-addr.arpa.' zone. The 'in-addr.arpa.' zone is a DNS zone (and, note, portion of the namespace) that holds a mapping from Internet addresses to names. This is the opposite (but not equal) transaction to normal whereby we look up a name to find an address. This can be useful in a number of ways and is detailed better in domain name system.
Technical details
The above is not very specific about the technical details of what a zone is. This is partly due to the non-specific nature of a zone (i.e. administrative/technical), as well as the many different systems that manage these zones, and finally because of the extensive use of domain-style naming for totally unrelated functions. Not that the above are problems, per-se, but they do clutter the technical discussion with qualifications and clarifications that eventually make it entirely indigestible. As such we will stick to discussing the technical details in respect of DNS zones in their primary function (administration of DNS namespace) and use (serving Internet addresses to requesting systems). Where technical examples or details are required we will again restrict ourselves to the most common DNS system ISC's BIND.
