A Uniform Resource Identifier (URI) is a compact string of characters for identifying an abstract or physical resource (Berners-Lee ta al, 1998, rfc2396). Breaking this down into manageable chunks:

• URIs are uniform: they always use or conform to specific Internet Engineering Taskforce (IETF) URI syntax
• URIs identify a given resource.

Consequently: at their most basic level, URIs use a consistent syntax and identify a resource. Though multiple URIs can identify a given resource simultaneously, it is imperative that a single URI should not identify multiple different resources. Hence, each URI should be globally unique.

Generic IETF URI syntax may be found here; World Wide Web-specific IETF syntax may be found here.

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