What is a subdomain?

A subdomain is a name in front of your domain that points somewhere of its own - shop.example.com.au, mail.example.com.au, portal.example.com.au:

• It is a separate destination - often a different server, and sometimes a different supplier entirely.
• It inherits the brand, not the security - a subdomain can be misconfigured while the main site is fine.
• They accumulate quietly - staging sites, old campaign pages and retired apps rarely get removed.

List every subdomain you have before you assume your website is your whole attack surface.