Every domain name and every IP range is registered somewhere. WHOIS is the old protocol for retrieving that registration: you send a line of text to a server and get a slab of text back, formatted differently by every registry.
RDAP is the successor
The Registration Data Access Protocol does the same thing over HTTPS and returns structured JSON. That makes it possible to label a field rather than guess which piece of text means what. All five RIRs and most domain registries now offer it; this site uses RDAP wherever it exists.
Registry, registrar and network
- Registry data — what the operator of the extension records; for .nl that is SIDN
- Registrar data — what the party you bought the domain from records
- Network registration — what an RIR such as RIPE NCC records about an IP range
What the GDPR does here
Personal details of private registrants are redacted by European registries. If a .nl domain shows no name and address, that is not a gap in the data: it is the law working as intended. For business registrants and for IP ranges the organisation is usually still listed, because that is not personal data.
What an IP registration does not say
For an IP range it names whoever operates the range — a provider, a hosting company, an institution. Not who is using the address right now. Those two are often confused, and the difference is exactly the difference between a network and a person.