Every IP address block is registered with a regional registry (RIPE NCC, ARIN, APNIC, LACNIC or AFRINIC). This tool queries these registries directly (RDAP protocol) to tell you which organisation an IP address belongs to.
Every domain name is registered with a registrar, itself tied to the registry for its extension (.com, .org, .net...). This tool queries these registries directly (RDAP protocol) to tell you who owns a domain name and until when it's reserved.
What is WHOIS?
WHOIS is a public service that shows who an IP address or a domain name is registered to: the holding organisation, network range or registrar, dates, and most importantly a contact to reach in case of abuse (spam, attack). This tool queries the RDAP protocol, the modern, structured version of the historical WHOIS.