In the Domain Name Server (DNS) hierarchy, a subdomain is a domain that is part of a larger domain name. A DNS hierarchy comprises of the top root-level domain, the top-level below followed by second-level domains and lastly subdomains.

Valid subdomains as described by RFC 1034 may contain any letter or number as well as the ‘-‘ (hyphen), but must begin with an alphabet character. While both upper and lower case characters are valid. Subdomains are case insensitive so ‘ABC.com’ is identical to ‘abc.COM’.

Length of sub domain
The length of a specific subdomain label is limited to 63 characters, but there is an arbitrary amount of subdomain labels in a domain name.

Structure
Sub-Domain are divided by dots.” “and read in descending specificities from left to right. Each subdomain includes all prior subdomains and any subdomains that follow. All subdomains are contained by the second-level domain, which is in turn contained by its top-level domain.

In this example the top-level domain is ‘tld’ and the second-level domain is ‘domain’. The Sub-Domain is enclosed within the ‘ domain ‘ in a tree structure.

Popular real-life examples would be news.google.com, mail.yahoo.com and en.wikipedia.org. In which news, mail and en are subdomains, respectively click to learn more about Subdomains.