FQDN (Fully Qualified Domain Name)

What is FQDN (Fully Qualified Domain Name)?

An FQDN (Fully Qualified Domain Name) is the complete, specific address of a website, including all parts of the domain hierarchy. Unlike partial domain names, an FQDN provides the exact location of a resource on the internet. For SSL certificates, FQDNs are crucial because certificates must match the exact domain name being accessed to establish trust.

FQDN Structure and Components

A complete FQDN consists of several hierarchical components:

• Hostname: The specific server or service name (like “www” or “mail”)
• Domain Name: The registered domain (like “example”)
• Top-Level Domain: The domain extension (like “.com” or “.org”)
• Root Domain: The implicit final dot in the hierarchy
• Complete Format: Results in names like “www.example.com.”

SSL Certificate Validation

SSL certificates use FQDNs to determine which domains they can secure. The certificate’s Common Name or Subject Alternative Names must exactly match the FQDN being accessed. Browsers perform strict FQDN matching during certificate validation, rejecting connections where the requested domain doesn’t match the certificate. This precision ensures that certificates can only be used for their intended domains, preventing unauthorized use and maintaining the security of the PKI system.