protocols to leverage a pre-existing, large, and widely-used
         set of resource identifiers.

      Resource
         A resource can be anything that has identity.  Familiar
         examples include an electronic document, an image, a service
         (e.g., "today's weather report for Los Angeles"), and a
         collection of other resources.  Not all resources are network
         "retrievable"; e.g., human beings, corporations, and bound
         books in a library can also be considered resources.




 
RFC 2396                   URI Generic Syntax                August 1998


         The resource is the conceptual mapping to an entity or set of
         entities, not necessarily the entity which corresponds to that
         mapping at any particular instance in time.  Thus, a resource
         can remain constant even when its content---the entities to
         which it currently corresponds---changes over time, provided
         that the conceptual mapping is not changed in the process.

      Identifier
         An identifier is an object that can act as a reference to
         something that has identity.  In the case of URI, the object is
         a sequence of characters with a restricted syntax.

   Having identified a resource, a system may perform a variety of
   operations on the resource, as might be characterized by such words
   as `access', `update', `replace', or `find attributes'.

1.2. URI, URL, and URN

   A URI can be further classified as a locator, a name, or both.  The
   term "Uniform Resource Locator" (URL) refers to the subset of URI
   that identify resources via a representation of their primary access
   mechanism (e.g., their network "location"), rather than identifying
   the resource by name or by some other attribute(s) of that resource.
   The term "Uniform Resource Name" (URN) refers to the subset of URI
   that are required to remain globally unique and persistent even when
   the resource ceases to exist or becomes unavailable.

   The URI scheme (Section 3.1) defines the namespace of the URI, and
   thus may further restrict the syntax and semantics of identifiers
   using that scheme.  This specification defines those elements of the
   URI syntax that are either required of all URI schemes or are common
   to many URI schemes.  It thus defines the syntax and semantics that
   are needed to implement a scheme-independent parsing mechanism for
   URI references, such that the scheme-dependent handling of a URI can
   be postponed until the scheme-dependent semantics are needed.  We use
   the term URL below when describing syntax or semantics that only
   apply to locators.

   Although many URL schemes are named after protocols, this does not
   imply that the only way to access the URL's resource is via the named
   protocol.  Gateways, proxies, caches, and name resolution services
   might be used to access some resources, independent of the protocol
   of their origin, and the resolution of some URL may require the use
   of more than one protocol (e.g., both DNS and HTTP are typically used
   to access an "http" URL's resource when it can't be found in a local
   cache).






 
RFC 2396                   URI Generic Syntax                August 1998


   A URN differs from a URL in that it's primary purpose is persistent
   labeling of a resource with an identifier.  That identifier is drawn
   from one of a set of defined namespaces, each of which has its own
   set name structure and assignment procedures.  The "urn" scheme has
   been reserved to establish the requirements for a standardized URN
   namespace, as defined in "URN Syntax" [RFC2141] and its related
   specifications.

   Most of the examples in this specification demonstrate URL, since
   they allow the most varied use of the syntax and often have a
   hierarchical namespace.  A parser of the URI syntax is capable of
   parsing both URL and URN references as a generic URI; once the scheme
   is determined, the scheme-specific parsing can be performed on the
   generic URI components.  In other words, the URI syntax is a superset
   of the syntax of all URI schemes.

1.3. Example URI

   The following examples illustrate URI that are in common use.

   ftp://ftp.is.co.za/rfc/rfc1808.txt
      -- ftp scheme for File Transfer Protocol services

   gopher://spinaltap.micro.umn.edu/00/Weather/California/Los%20Angeles
      -- gopher scheme for Gopher and Gopher+ Protocol services