FQDN
Fully Qualified Domain Name — an absolute DNS name that includes every label up to the root.
An FQDN (Fully Qualified Domain Name) is an absolute name in DNS: it lists every label from the host all the way up to the root and is terminated by an implicit (or explicit) trailing dot. www.example.com. is an FQDN. www on its own, or even www.example, is a partial name that only becomes resolvable once the resolver appends a search suffix.
The structure is hierarchical and read right-to-left. The rightmost label is the TLD (com), the next is the registered domain (example), and labels to the left are subdomains administered by the zone owner (www). Each label is limited to 63 octets and the whole name to 255 octets including length bytes. Comparison is case-insensitive: Example.COM. and example.com. are the same name.
The trailing dot matters in zone files and in tools that take raw DNS input. Without it, software may treat the name as relative and append a configured search domain — a common cause of the wrong host being queried. dig www.example.com and dig www.example.com. should return the same data, but dig www will not, because the resolver appends whatever search list is configured on the system.
An FQDN is what an A-record, AAAA, CNAME or MX actually answers for. TLS certificates are issued to FQDNs (or wildcards that cover them), reverse DNS returns FQDNs in PTR answers, and configuration files in mail and web servers normally expect the fully qualified form rather than a short host name.
Key Points
- Absolute DNS name including every label up to the root
- Terminated by an implicit or explicit trailing dot
- Hierarchy read right-to-left: TLD → domain → subdomains
- Label length ≤ 63 octets, total name ≤ 255 octets
- Case-insensitive comparison
- Partial names get a search suffix appended by the resolver
- Used as the subject of
A,AAAA,CNAME,MXand TLS certificates