Fully Qualified Domain Name (FQDN) or Absolute Domain Name (ADN)

• contains both a host name and a domain name (which are separated by a period)
• a domain name that includes the top-level domain
• example: a device with the hostname myhost in the parent domain example.com has the fully qualified domain name myhost.example.com

Partially Qualified Domain Name (PQDN) or Relative Domain Name (RDN)

• a domain name that doesn’t include the top-level domain
• types of PQDN:
  • Host Name or Hostname
    • the left-most label in a FQDN
    • the host name represents the network or system used to deliver a user to a certain address or location
  • Domain Name
    • the domain name represents the site or project that the user is accessing

related to: URL, URI, and URN

FQDN Syntax

A fully qualified domain name consists of a list of domain labels representing the hierarchy from the lowest relevant level in the DNS to the top-level domain (TLD). The domain labels are concatenated using the full stop “.” character (dot or period) as a separator between labels.

The DNS root is unnamed, expressed as the empty label terminated by the dot. This is most notable in DNS zone files in which a fully qualified domain name must be specified with a trailing dot. For example, somehost.example.com. explicitly specifies an absolute domain name that ends with the empty top level domain label.