What is a subdomain?

Subdomain is a domain that is a part of a main domain. For example, abc.example.com. Here 'abc' is the subdomain and 'example.com' is the main domain. You can also add multiple levels of subdomains. Subdomains are different from directories. For example, example.com/abc points to a directory within the example.com domain, not to a subdomain of example.com.

The whole domain name does not exceed a total length of 255 characters, but in practice most domain registries limit at 253 characters.