1. What's Reverse DNS lookup(rDNS)
Typically, the Domain Name System is used to determine what IP address is associated with a given domain name. So, to reverse-resolve a known IP address is to look up what host and domain name belongs to that IP address.
A reverse lookup is often referred to as reverse resolving, or more specifically reverse DNS lookup, and is accomplished using a "reverse IN-ADDR entry" in the form of a PTR record.
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2. What's Reverse DNS lookup(rDNS)
WHOIS (pronounced "who is"; not an acronym) is a query/response protocol which is widely used for querying an official database in order to determine the owner of a domain name, an IP address, or an autonomous system number on the Internet. WHOIS lookups were traditionally made using a command line interface, but a number of simplified web-based tools now exist for looking up domain ownership details from different databases. Web-based WHOIS clients still rely on the WHOIS protocol to connect to a WHOIS server and do lookups, and command-line WHOIS clients are still quite widely used by system administrators. WHOIS normally runs on TCP port 43.
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