What is a subdomain?¶
A subdomain is an additional name placed before the registered domain. It can identify a particular service or area, but it normally does not have a separate registrant from the main domain.
In login.example.com, login is the subdomain and example.com is the registered domain.
Why it matters¶
Organisations use subdomains to separate services such as:
- public websites;
- webmail;
- account portals;
- payment services;
- support systems; and
- third-party applications.
Two subdomains beneath the same registered domain may point to different IP addresses, providers or management teams.
Identify the real registered domain¶
A common mistake is to trust the first familiar word in an address. Control is normally identified from the right-hand end of the hostname.
For example:
bank.example-login.com
Here, bank is only a subdomain of example-login.com. Its position does not mean that the address is controlled by a bank.
Long hostnames may contain several subdomain levels. Identify the registered domain correctly before deciding which provider may hold records.
Preserve the complete hostname¶
Do not shorten login.example.com to example.com in notes or requests. The subdomain may be essential to identifying the specific service.
Control of the main domain may be relevant, but it does not prove who created or used the subdomain. It could have been configured by an employee, contractor, customer, attacker or automated service.
Key takeaway
Record the complete hostname, identify the true registered domain, and do not let a familiar word at the beginning of an address mislead you.