What is a subdomain?¶

A subdomain is a hostname placed before the registered domain, such as login.example.com. It can identify a particular service without being a separately registered domain.

Why it matters¶

Different subdomains may point to different providers, applications or teams:

• public website;
• account portal;
• webmail;
• payment service; or
• third-party application.

The complete hostname is therefore important.

Identify the registered domain correctly¶

In bank.example-login.com, “bank” is only part of a hostname; the registered domain is example-login.com. Work from the right-hand end and confirm the relevant registry structure. Do not let a familiar word at the beginning imply affiliation.

A subdomain may be configured by an employee, contractor, customer or attacker. Control of the parent domain does not prove who used the service.