James's Ramblings

DNS and BIND

Created: August 17, 2019

The Domain Namespace

  • The domain namespace is a large inverted tree containing all possible domain names and subdomains – which are represented by branches.
  • The root is the top level of the inverted tree.
  • The depth of the tree is limited to 127 levels.

Domain Names

  • Each node in the tree has a text label (without dots) that can be up to 63 characters long.
  • A null label is reserved for the root.
  • When the root node’s label appears in a node’s domain name, it appears as a dot on the end, for example “reddit.com.”
  • When the root node’s label appears by itself, it is written as a single dot “.”
  • Some software interprets a trailing dot in a domain name to indicate that the domain name is absolute.
  • An absolute domain name is also called a fully qualified domain name (FQDN).
  • Domain names without trailing dots are sometimes interpreted as relative to some domain name other than root.
  • DNS requires that sibling nodes – nodes that are children of the same parent - have different labels.

Domains

  • A domain is a subtree of the domain namespace.
  • The domain name of a domain is the same as the domain name of the node at the very top of the domain.
  • DNS domains are not related to domains in Sun’s Network Information Service (NIS) protocol, nor are they related to NT domains.
  • Active Directory domains ARE closely related to DNS domains.
  • A subdomain is a subtree of a domain.
  • A top-level domain is a child of the root.
  • A first-level domain is a child of the root.
  • A second-level domain is a child of a first-level domain, and so on.

Resource Records

  • The data associated with domain names is contained in resource records, or RRs.
  • Records are divided into classes, each of which petains to a type of network or software.
  • Currently, there are classes for internets, networks based on the Chaosnet protocols and networks that use Hesiod software.

Top-Level Domains

  • The original seven top-level domains com, edu, gov, mil, net, org and int are now called generic top-level domains or gTLDs.
  • There is also an arpa domain that was originally used for ARPAnet’s transition from host tables to DNS. It still exists but is used for something else.
  • There are country-code top-level domains that follow the international standard ISO-3166.
  • Later came sponsored TLDs (sTLDs) for special organisations, e.g. aero, coop and museum.
  • At the same time, unsponsored gTLDs, such as info and biz, were also created.