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A DNS zone is a specific portion of the DNS namespace in the
Domain Name System The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned t ...
(DNS), which is managed by a specific organization or administrator. A DNS zone is an administrative space that allows for more granular control of the DNS components, such as authoritative nameserver. The DNS is broken up into many different zones, which are distinctly managed areas in the DNS namespace. DNS zones are not necessarily physically separated from one another, however, a DNS zone can contain multiple
subdomain In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is a part of another (main) domain. For example, if a domain offered an online store as part of their website example.com, it might use the subdomain shop.example.com . ...
s and multiple zones can exist on the same server. The domain namespace of the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, p ...
is organized into a hierarchical layout of subdomains below the DNS root domain. The individual domains of this tree may serve as delegation points for administrative authority and management. However, usually, it is furthermore desirable to implement fine-grained boundaries of delegation, so that multiple sub-levels of a domain may be managed independently. Therefore, the domain name space is partitioned into areas (''zones'') for this purpose. A zone starts at a domain and extends downward in the tree to the leaf nodes or to the top-level of subdomains where other zones start. A DNS zone is implemented in the configuration system of a domain name server. Historically, it is defined in the
zone file A Domain Name System (DNS) zone file is a text file that describes a DNS zone. A DNS zone is a subset, often a single domain, of the hierarchical domain name structure of the DNS. The zone file contains mappings between domain names and IP add ...
, an operating system
text file A text file (sometimes spelled textfile; an old alternative name is flatfile) is a kind of computer file that is structured as a sequence of lines of electronic text. A text file exists stored as data within a computer file system. In operat ...
that starts with the special DNS record type ''Start of Authority'' (SOA) and contains all records for the resources described within the zone. This format was originally used by the Berkeley Internet Name Domain Server (BIND) software package and is defined in RFC 1034 and RFC 1035.


Domains and zones

Most top-level
domain name registry A domain name registry is a database of all domain names and the associated registrant information in the top level domains of the Domain Name System (DNS) of the Internet that enables third party entities to request administrative control of a d ...
operators offer their name spaces to the public or to entities with mandated geographic or otherwise scoped purpose for registration of second-level domains. Similarly an organization in charge of a lower level domain may operate its name space similarly and subdivide its space. Each registration or allocation of subdomain space obligates the registrant to maintain an administrative and technical infrastructure to manage the responsibility for its zone, including sub-delegation to lower-level domains. Each delegation confers essentially unrestricted technical autonomy over the allocated space. An area of one or more subdomains that has been delegated for management is called a DNS zone. A zone always starts at a domain boundary to include all leaf nodes (hosts) in the domain, or it ends at the boundary of another independently managed zone. As each domain is further divided into sub-domains, each becoming a DNS zone itself with its own set of administrators and DNS servers, the tree grows with the largest number of leaf nodes at the bottom. At this lowest level, in the end-nodes or leaves of the tree, the term ''DNS zone'' becomes essentially synonymous with the term "
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined ** Domain of definition of a partial function ** Natural domain of a partial function **Domain of holomorphy of a function * ...
", both in terms of use and administration. The term ''domain'' is used in the business functions of the entity assigned to it and the term ''zone'' is usually used for configuration of DNS services.


Forward DNS zones

DNS zones contain the records for the mapping of domain names to IP addresses or other information. The resolution of a domain name to its assigned information is also referred to as ''forward'' resolution and the DNS zones associated with such processes are often referred to as ''forward'' zones. The term arose as the opposite of ''reverse'' zones, which are used for the reverse process: finding the DNS name associated with an IP address. Such reverse zones are maintained in the Internet Address and Routing Parameter Area (domain arpa). Another common use of the term ''forward zone'' refers to a specific configuration of DNS
name server A name server refers to the server component of the Domain Name System (DNS), one of the two principal namespaces of the Internet. The most important function of DNS servers is the translation (resolution) of human-memorable domain names (example ...
s, particularly
caching name server A name server refers to the server component of the Domain Name System (DNS), one of the two principal namespaces of the Internet. The most important function of DNS servers is the translation (resolution) of human-memorable domain names (example. ...
s, in which resolution of a domain name is forwarded to another name server that is authoritative for the domain in question, rather than being answered from the established cache memory.


Zones for Internet infrastructure

The top-level domain arpa serves as a delegation zone for various technical infrastructure aspects of DNS and the Internet, and does not implement the registration and delegation system of the country and generic domains. The name ''arpa'' is a remnant of the
ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical fou ...
, one of the predecessor stages of the Internet. Intended as a transitional aid to the DNS, deleting the domain arpa was later found to be impractical. Consequently, the name was officially redefined as an acronym for ''Address and Routing Parameter Area''. It contains sub-zones used for reverse resolution of IP addresses to host names (
IPv4 Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version d ...
: in-addr.arpa,
IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. I ...
: ip6.arpa), telephone number mapping (
ENUM Telephone number mapping is a system of unifying the international telephone number system of the public switched telephone network with the Internet addressing and identification name spaces. Internationally, telephone numbers are systematic ...
, e164.arpa), and uniform resource identifier resolution (uri.arpa, urn.arpa). Although the administrative structure of this domain and its sub-domains is different, the technical delegation into zones of responsibility is similar and the DNS tools and servers used are identical to any other zone. Sub-zones are delegated by components of the respective resources. For example, 8.8.2.5.5.2.2.0.0.8.1.e164.arpa., which might represent an E.164 telephone number in the
ENUM Telephone number mapping is a system of unifying the international telephone number system of the public switched telephone network with the Internet addressing and identification name spaces. Internationally, telephone numbers are systematic ...
system, might be sub-delegated at suitable boundaries of the name. An example of an
IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
es in the reverse DNS zone is 166.188.77.208.in-addr.arpa, which represents the address 208.77.188.166 and resolves to the domain name ''www.example.com''. In the case of IP addresses, the reverse zones are delegated to the
Internet service provider An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise priva ...
(ISP) to which the IP address block is assigned. When an ISP allocates a range to a customer, it usually also delegates the management of that space to the customer by insertion of name server resource records pointing to the customer's DNS facilities into their zone, or provides other management tools. Allocations of single IP addresses for networks connected through network address translation (NAT) typically do not provide such facilities.


Example of zone authority in DNS queries

As an example of the DNS resolving process, consider the role of a recursive
DNS resolver The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned t ...
attempting to look up the address "en.wikipedia.org.". It begins with a list of addresses for the most authoritative name servers it knows about – the root zone name servers (indicated by the full stop or period), which contains name server information for all
top-level domain A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain. The top-level domain names are installed in the root zone of the name space. For all domains in ...
s (TLDs) of the Internet. When querying one of the root name servers, it is possible that the root zone will not directly contain a record for "en.wikipedia.org.", in which case it will provide a referral to the authoritative name servers for the "org."
top-level domain A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain. The top-level domain names are installed in the root zone of the name space. For all domains in ...
(TLD). The resolver is issued a referral to the authoritative name servers for the "org." zone, which it will contact for more specific information. Again when querying one of the "org." name servers, the resolver may be issued with another referral to the "wikipedia.org." zone, whereupon it will again query for "en.wikipedia.org.". Since () "en.wikipedia.org." is a
CNAME A Canonical Name record (abbreviated as CNAME record) is a type of resource record in the Domain Name System (DNS) that maps one domain name (an alias) to another (the canonical name). This can prove convenient when running multiple services (l ...
to "text.wikimedia.org." (which is in turn a CNAME to "text.esams.wikimedia.org."), and the "wikipedia.org." name servers also happen to contain authoritative data for the "wikimedia.org." zone, the resolution of this particular query occurs entirely within the queried name server, and the resolver will receive the address record it requires with no further referrals. If the last name server queried did not contain authoritative data for the target of the CNAME, it would have issued the resolver with yet another referral, this time to the zone ''text.wikimedia.org.''. However, since the resolver had previously determined the authoritative name servers for the zone ''org.'', it does not need to begin the resolution process from scratch but instead start at zone ''org.'', thus avoiding another query to the root name servers. There is no requirement that resolving should involve any referrals at all. Looking up ''en.wikipedia.org.'' on the root name servers always results in referrals, but if an alternative DNS root is used which is set up to contain a record for ''en.wikipedia.org.'', then the record is returned on the first query.


See also

*
Domain propagation The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned ...
* DNS zone transfer


References

{{DEFAULTSORT:Dns Zone Domain Name System