The mapping for URIs onto some existing standard and experimental protocols is outlined in the BNF syntax definition. Notes on particular protocols follow. These URIs are frequently referred to as URLs, though the exact definition of the term URL is still under discussion (March 1993). The schemes covered are:
The following schemes are proposed as essential to the unification of the web with electronic mail, but not currently (to the author's knowledge) implemented:
The schemes for X.500, network management database, and Whois++ have not been specified and may be the subject of further study. Schemes for Prospero, and restricted NNTP use are not currently implemented as far as the author is aware.
The "urn" prefix is reserved for use in encoding a Uniform Resource Name when that has been developed by the IETF working group.
New schemes may be registered at a later time.