How it works — and what it can't do
The tool looks up a fixed list of common names (such as www, mail, api, cdn, dev and staging) under the domain and lists the ones that resolve to an IPv4 address.
It does not scan the network, read certificate logs or guess thousands of names, so it won't find every subdomain. Use it to audit your own domain's common entries — for example, forgotten staging hosts that should be removed.
Questions
Is it legal to check subdomains?
Looking up public DNS records is ordinary DNS usage. Only test systems you're authorised to test beyond that.