fqdn

A fully qualified domain name (FQDN) is the complete domain name of a specific host in the hierarchical Domain Name System (DNS). It uniquely identifies a host on the Internet or a private network. An FQDN consists of a sequence of labels separated by dots, beginning with the host name and ending with the top-level domain, and optionally ending with a trailing dot that represents the root of the DNS.

For example, "server1.example.com." has "server1" as the host label, "example" as the second-level domain, "com" as the top-level domain, and the trailing dot marks the root. In practice, many systems omit the trailing dot, treating it as implied. The FQDN specifies the exact path from the DNS root to the host and is distinct from a bare hostname, which may be valid only within a local context.

Resolution of an FQDN involves DNS queries that start at the root servers and proceed to the appropriate top-level and authoritative servers until the host’s IP address is returned. FQDNs can be used in public DNS and in private DNS zones; on private networks, they often map to internal hosts and services according to local DNS policies.

Uses and standards: FQDNs are essential for secure connections via TLS certificates, where the certificate must include the FQDN as a subject name or in the Subject Alternative Name field. They are also used in email routing and various network services that require an unambiguous host identifier. Constraints include a maximum of 63 characters per label and about 253 characters for the full name in its textual representation; the optional trailing dot denotes the DNS root.