Whois is a service which can provide information about domain names. Domains are given out by registrars, and information about them is usually public because registrars charge extra for private registration.

In order to function, whois needs two things - a domain name to look up and a whois server. The whois server is a database which is periodically updated with information from various registrars about the domains associated with them.

The command itself is very simple.

whois <domain name>
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As we can see, whois yielded information about the domain name's registrar, the time of creation, the time of the last update and much more. In fact, example.com uses private registration so this information is actually not that much. When the domain is publicly registered, a whois look-up can provide information such as the phone number, email address, ISP and country of residence of the person / organisation that owns the domain, additional domains owned by the same organisation as well as email servers.

It is also possible to specify a custom whois server with the -h flag.

whois <domain name> -h <whois server>
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whois is also capable of obtaining information from an IP address.

whois <ip>
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This is the result from the reverse whois lookup for the IP address of example.com. The reverse lookup provides us with information about who is hosting the IP. This time it yielded a person's name, an address and a phone number. Looking these up on Google, we see that they are actually associated with a physical office of edg.io.

One should ways do both a normal as well as a reverse whois lookup because on might reveal information that the other does not.
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