Fully qualified domain name
A fully qualified domain name (FQDN), sometimes also referred to as an absolute domain name, is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain and the root zone. A fully qualified domain name is distinguished by its lack of ambiguity in terms of DNS zone location in the hierarchy of DNS labels: it can be interpreted only in one way.
Domain names in DNS are read from right to left, therefore we need to know where to start from to find the explicit location of a host using a domain name. Using this information we can see how DNS requests/lookups (DNS zone checks) are processed as follows:
- The root zone, being at the top of the hierarchy, is represented simply by a full stop (period), as it is unnamed and therefore has no label. Typical DNS will use the long-established root zone which is coordinated by IANA (a department of ICANN).
- Under the root zone is the Top Level Domain (TLD).
- Under these is a Second Level Domain (SLD), which is a subdomain of the TLD above it.
- Under these can be a Third Level Domain, yet another subdomain of the SLD above it.
Each subdomain is a child domain of every domain above it (its parent domain) and separated by a full stop. With the final full stop representing the root zone. Due to the lack of a label/name for this zone, it is often not represented in many softwares such as web browsers, resulting in a DNS hierarchy ending with the top-level domain with no trailing full stop. However, this is actually only a partially qualified domain name (PQDN), since we can fairly safely assume that we want to be using the same, agreed upon, root zone to interact with the domain we want (though there are alternate root zones).
In cases where the FQDN is critical, such as within DNS zone file/server records, the full stop (period) character is required to form the fully qualified domain name.
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 root zone. The domain labels are concatenated using the full stop “
.” character (dot or period) as a separator between labels. The length of each label must be between 1 and 63 octets, and the full domain name is limited to 255 octets, full stops included.
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.
A device with the hostname
somehost in the parent domain
example.com has the fully qualified domain name
somehost.example.com.. The FQDN uniquely distinguishes the device from any other hosts called
somehost in other domains.
Many DNS resolvers process a domain name that contains a dot in any position as being fully qualified or add the final dot needed for the root of the DNS tree. Resolvers process a domain name without any dots as unqualified and automatically append the system's default domain name and the final dot. The DNS packet does not have any dots though, it uses the length of the domain as a separator and ends in 0x00.
Some applications, such as web browsers, try to resolve the domain name part of a uniform resource locator (URL) if the resolver cannot find the specified domain or if it is clearly not fully qualified by appending frequently used top-level domains and testing the result. Some applications, however, never use trailing dots to indicate absoluteness, because the underlying protocols require the use of FQDNs, such as the Simple Mail Transfer Protocol (SMTP).
Partially qualified domain name
A partially qualified domain name does not include all labels to the DNS root. Such a name is also known as a relative domain name. Relative domain names are often simply hostnames, i.e. the left-most label in a fully qualified name.
- Mockapetris, Paul. Domain names – Implementation and Specification. doi:10.17487/RFC1035. RFC 1035.
- April N. Marine; Joyce K. Reynolds; Gary Scott Malkin (March 1994). "Questions About the Domain Name System". Answers to Commonly asked "New Internet User" Questions. IETF. sec. 5. doi:10.17487/RFC1594. RFC 1594. Retrieved 29 April 2013.
If you think of the DNS as a tree-structure with each node having its own label, a fully qualified domain name for a specific node would be its label followed by the labels of all the other nodes between it and the root of the tree.
- RFC 2181
- Fisher, Tim. "FQDN". About.com. Retrieved 20 March 2013.
- "Wireshark Q&A". osqa-ask.wireshark.org. Retrieved 13 April 2021.
- "Definition of domain names in Simple Mail Transfer Protocol". Tools.ietf.org. 1998-05-21. Retrieved 2014-01-08.
- "A Security Problem and Proposed Correction With Widely Deployed DNS Software".
- "Fully Qualified Domain Name (FQDN) and Partially Qualified Domain Name (PQDN)".
- RFC 1123: Requirements for Internet Hosts – application and support
- RFC 1535: A Security Problem and Proposed Correction With Widely Deployed DNS Software
- RFC 2181: Clarifications to the DNS specification
- RFC 2826: IAB Technical Comment on the Unique DNS Root
- RFC 4703: Resolution of Fully Qualified Domain Name (FQDN) Conflicts among Dynamic Host Configuration Protocol (DHCP) Clients