In the Domain Name Server (DNS) hierarchy, a subdomain is a domain that is part of a larger domain name. A DNS hierarchy consists of a "root" level domain at the top, below which are Top-Level-Domains (TLDs), followed by Second-Level-Domains and finally Subdomains.
Valid subdomains according to RFC 1034 can contain any letters and numbers including the conjunction "-", but must start with a letter of the alphabet. Uppercase and lowercase letters have an effect.
Subdomain Length
The length of subdomain labels is limited to 63 characters, but the number of subdomain labels in a domain name is subject to change.
Structure of
Subdomains are separated by a period "." and read from left to right. Each subdomain contains all previous subdomains, and those contained by subsequent subdomains. All subdomains are contained by the Second-Level-Domain, and followed by the Top-Level-Domain.
Common everyday examples are news.google.com, mail.yahoo.com, support.exabytes.com and en.wikipedia.org, usually in news and email sites, subdomains are always used.