URI: Uniform Resource Identifier
URI identifies a logical or physical resource on the web. URL and URN are subtypes of URI.

• URL locates a resource
• URN names a resource A URI is composed of the following parts: scheme:[//authority]path[?query][#fragment]

URL: Uniform Resource Locator
URL is the key concept of HTTP. It is the address of a unique resource on the web. It can be used with other protocols like FTP and JDBC.

URN: Uniform Resource Name
URL uses the urn scheme. URNs cannot be used to locate a resource. A simple example given in the diagram is composed of a namespace and a namespace-specific string.

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