Subdomainlevel

Subdomainlevel is a term used to describe the depth of a subdomain within the Domain Name System (DNS). It refers to how many levels of labels precede the registered domain, or apex, portion of a fully qualified domain name. In practice, the leftmost labels indicate successive subdomains, each adding a level of hierarchy.

In typical usage, subdomain level is counted relative to the registrable domain. For example, in www.example.com, the label www is a first-level subdomain under the registered domain example.com. In a.google.mail.example.com, there are multiple subdomain levels (google, mail, and example) before the core domain. The exact interpretation can depend on how the registrable domain is defined, which is influenced by public suffix policies that determine what constitutes the apex of a domain.

DNS constraints and considerations. The DNS protocol allows up to 127 levels of labels, with individual labels limited to 63 characters and the whole domain name generally capped at 253 to 255 characters. In practice, most subdomains are shallow (one to three levels), though deeper levels are technically possible. The concept of subdomain levels is also relevant for SSL/TLS certificates, where wildcards (*.example.com) cover only a single label beneath the base domain, not multi-level subdomains.

Applications and implications. Subdomainlevel affects DNS management, certificate issuance, and branding or routing decisions. Services may impose practical limits on subdomain depth, and organizations often organize subdomains to reflect organizational structure, geographic regions, or product lines.

Note that the exact depth interpretation can vary with the public suffix list and how a domain’s registrable boundary is defined for a given context.