FQDNs

FQDN stands for Fully Qualified Domain Name. It is a domain name that precisely specifies a host’s location in the hierarchical Domain Name System (DNS) and ends with the root label. In practice, many systems display FQDNs without the terminating dot, but the trailing dot denotes the root in DNS notation.

An FQDN is composed of a sequence of labels separated by dots. The rightmost label is the top-level domain (TLD) such as com, net, org; to the left are second-level domains and subdomains, with the leftmost label identifying the host. Each label may contain letters, digits, and hyphens and must start and end with a letter or digit. Individual labels are up to 63 characters, and the total length of an FQDN is limited to 253 characters. The full sequence ends with a trailing dot (the root), indicating it is absolute.

DNS resolution uses the hierarchical structure: root servers delegate to TLD servers, which delegate to authoritative servers for a domain. A host name like www.example.com. resolves to an IP address via A (IPv4) or AAAA (IPv6) records, fetched by recursive resolvers and cached by DNS caches. FQDNs disambiguate hosts across the global namespace; a shorter label like www.example.com without the trailing dot is often treated as a relative name in certain contexts.

Practical use: FQDNs identify servers in TLS certificates (CN or SAN fields), in configuration files, and in email routing (MX records). They differ from general domain names in that an FQDN explicitly includes the full path to the root, ensuring uniqueness in the DNS. Security considerations include DNSSEC to authenticate responses and prevent spoofing.