subdomains

Subdomains are subdivisions of a domain in the Domain Name System (DNS) that use an additional leftward label to create a distinct host within a domain. For example, blog.example.com is a subdomain of example.com. Subdomains are commonly used to organize services, environments, or geographic or linguistic variations while sharing the same registered domain.

Subdomains are defined by DNS records under the parent domain. An A or AAAA record maps the subdomain to an IP address, a CNAME record can alias it to another hostname, and NS records can delegate authority for the subdomain to a separate DNS zone. A subdomain may remain in the parent zone or be delegated to its own authoritative DNS servers.

Delegation and zones: If a subdomain is delegated, the parent zone contains NS records pointing to the subdomain’s authoritative servers, and the subdomain maintains its own DNS zone with its own records. If not delegated, the subdomain’s records reside in the parent zone.

Typical uses include hosting different services (such as www, api, or mail for a domain), creating staging or development environments (dev.example.com), regional or language variants (eu.example.com, fr.example.com), and multitenant or branding configurations.

Subdomain versus subdirectory: A subdomain uses a distinct host name, while a subdirectory uses a path under the same host. Subdomains affect DNS, cookies, and TLS scoping, whereas subdirectories impact URL structure and SEO in different ways.

Security and management: TLS certificates may need to cover specific subdomains, and wildcard certificates can cover many subdomains. DNSSEC helps prevent spoofing. Best practices include careful namespace planning, appropriate TTLs, and monitoring DNS propagation.