How DNS works

A domain tree (or hierarchy) is what DNS hostnames are organised in. In each leaf in the tree, or each node, there are one or more resource records. These resource records hold information associated with the domain name. The tree is sub-divided into zones.

A zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. Several zones can be hosted with a single nameserver.

Control can be delegated to another administrator by a system 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.

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

A resolver looks up the information associated with nodes. By sending DNS requests and heeding DNS responses, a resolver knows how to communicate with name servers. 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.

In theory, this subdivision can go down to 127 levels deep, 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. In practice, some domain registries have shorter limits than that.

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

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

A recursive query is when the DNS server will fully answer the query (or give an error). 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.

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