Existing schemes

These are formalization of the addressing used by existing protocols which are important in the network information retrieval world.
FTP
the Internet File Transfer Protocol
Message identifiers
Mail messages have unique names.
Usenet news
Running over the Internet, usenet news articles are transferred with the network News Transfer Protocol.
Gopher
A popular CWIS protocol
WAIS
Naming Databases and documents
HTTP
Hypertext Transfer protocol is WWW's "native" protocol.
(up to Naming and Addressing)
Tim BL