In this scheme[4], the Element ID component of the URN is separated into two, a naming authority and a resource identifier, e.g.
dns:shells.unix.computing.subjects:zsh
The naming authorities are derived from Internet domain names, but unlike the x-dns-2 scheme, it is required that for a given naming authority such as lut.ac.uk, there be a host or alias for uri.lut.ac.uk. The computer which answers to this name is assumed to be running at least a minimal implementation of the whois++ protocol[27].
Whois++ is a general purpose Internet directory service which aims to provide roughly the same core functionality as X.500[28,29], but without the problems associated with use of the OSI architecture - cf. [30]. In practice this server only needs to understand enough of whois++ to be able to do simple lookups based on the URN.