Hi, does anyone know if it is illegal to look up companies on whois and contact them to potentially buy their domain names?

Thats one of the reason for the whois.

The whois service displays all available registration details for a domain including contact information for the registered owner. If you wish to make contact with a domain owner using this information then there shouldn't be a problem in doing so.

For all .uk and .eu domains where the domain is registered to an individual the registered owner details can and normally withheld from the public service. If a domain of this type is being used for business purposes then all register details will be available within the whois.

For top level domains, .com .net ect then all information will be available unless the registrar offers a privacy service where by the details will either be encrypted or show as the registrar.

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Domainmonster.com Customer Support Team.
Domain Name Registration Information & Resources.