In the Domain Name Server (DNS) structure, a subdomain refers to a domain that is a part of a broader domain name. The DNS hierarchy starts with the root-level domain at the highest point, followed by top-level domains, then second-level domains, and finally subdomains.
According to RFC 1034, valid subdomains can include any letters or numbers along with the '-' (hyphen), but they must start with a letter. Both uppercase and lowercase letters are acceptable; however, subdomains are case insensitive, meaning 'ABC.com' is the same as 'abc.COM'.
Subdomain Length
The length of each individual subdomain label is capped at 63 characters, though there is no limit to the number of subdomain labels that can exist within a domain name.
Structure
Subdomains are delineated by dots "." and are interpreted from left to right, showing decreasing specificity. Each subdomain includes all preceding subdomains and is encompassed by any following subdomains. All subdomains fall under the second-level domain, which is subsequently encompassed by its top-level domain.
Example:
subCBA.subBA.subA.domain.tld
In this instance, the top-level domain is 'tld', and 'domain' serves as the second-level domain. The subdomains are arranged in a tree structure within 'domain'.
Common real-world examples include news.google.com, mail.yahoo.com, support.exabytes.com, and en.wikipedia.org, where news, mail, and en are subdomains, respectively.