Sub-Domain

In the Domain Name Server (DNS) hierarchy, a sub-domain is a domain that is part of a broader domain name. A DNS hierarchy consists of the highest root-level domain, the first-level domain below followed by second-level domains and subdomains at last.

Valid sub-domains as defined in RFC 1034 that contain any letter or number as well as-‘ ‘(hyphen). Sub-domain must start with an alphabet character. Sub-domains are case-insensitive, and ‘ABC.com’ is the same as ‘abc.COM’ when the upper and lower case characters are real.

Length of sub domain
The length of a  sub-domain label is limited to 63 characters, although there is an arbitrary number of subdomain labels in the domain name.

Structure
“Dots” seperated the Sub-Domains and read in descending detail from left to right. It contains all of the previous subdomains and all of the following subdomains. Both subdomains are composed of a second-level domain, which in effect includes the top-level domain.

For this case, the top-level domain is ‘tld’ and the second-level domain is ‘domain.’The sub-domain enclosed in the tree structure within the ‘domain.’

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. Learn more.