How DNS works

DNS organises hostnames in a domain tree (or hierarchy). There are one or more resource records in each node or leaf in the tree, and these hold information associated with the domain name. The tree is sub-divided into zones.

A collection of connected nodes authoritatively served by an authoritative DNS nameserver are what a zone consists of. A single nameserver can host several zones.

A system administrator can delegate control to the other administrator, when that system administrator wants to let another administrator control a part of the domain name space within his or her zone of authority.

This splits a part of the old zone off into a new zone, which comes under the authority of the second administrator's nameservers. The old zone then becomes no longer authoritative for what goes under the authority of the new zone.

The information associated with nodes is looked up by the resolver. A resolver knows how to communicate with name servers by sending DNS requests, and heeding DNS responses. To find the needed information, Resolving usually entails iterating through several name servers.

Some resolvers can only communicate with a single name server as they function simplistically. These simple resolvers rely on a recursing name server to perform the work of finding information for them.

Parts of a domain name

A domain name usually consists of two or more parts (technically labels), separated by dots. For example ukfast.net.

The rightmost label conveys the top-level domain (for example, the address www.ukfast.net has the top-level domain net).

Each label to the left specifies a subdivision or subdomain of the domain above it. Relative dependence is expressed by “subdomain”, and not absolute dependence. For example, ukfast.net comprises a subdomain of the net domain, and www.ukfast.net comprises a subdomain of the domain ukfast.net.

This subdivision can go down to 127 levels deep in theory, and each label can contain up to 63 characters. This is as long as a whole domain does not exceed a total length of 255 characters. Some domain registries have shorter limits than that, in practice.

A hostname refers to a domain name that has one or more associated IP addresses. A hierarchical set of DNS servers is what the Domain Name System consists of.

Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it.

The hierarchy of domains matches the hierarchy of authoritative DNS servers. At the top of the hierarchy stand the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD).

Iterative and recursive queries:

When the DNS server may provide a partial answer to the query (or give an error), this is known as an Iterative query. Non-recursive queries must be supported by DNS servers.

When the DNS server will fully answer the query (or give an error), this is a recursive query. DNS servers are not required to support recursive queries and both the resolver (or another DNS acting recursively on behalf of another resolver) negotiate use of recursive service using bits in the query headers.

If you have any questions about DNS then visit our DNS FAQ section for more help.