The Shared WhoIs Project is a common exchange format used by regional internet registries such as InterNIC, ARIN and RIPE to trade IP address assignment information. SWIP is what allows you to query a DNS server with any given section of Class-C IP space using whois and get back a list of every user in said space. By "user" I mean the actual owners of the IP addresses which are usually companies, organizations, governments or other groups rather than individuals.

See also: X.500