URIs

URIs, or Uniform Resource Identifiers, are strings used to identify resources on the Internet and beyond. They encompass URLs (Uniform Resource Locators), which identify resources by their location, and URNs (Uniform Resource Names), which identify resources by a name within a namespace. The URI syntax is standardized in RFC 3986. A typical URI consists of a scheme, a hier-part, and an optional query and fragment.

From the overall syntax, a URI may include an authority component containing user information, a host, and an optional port; a path that identifies a resource within the scope of the host; and optional query and fragment components. The host is generally case-insensitive, while the path is often case-sensitive. Many characters are reserved for special meaning and must be percent-encoded when used in non-literal form.

IRI extension and encoding: URIs that use Unicode are called IRIs; they are transformed to URIs via percent-encoding. Unicode domains are handled via IDNA (Punycode).

Usage: URIs enable hyperlinks, web APIs, and metadata schemes (for example RDF). Dereferencing a URI uses the appropriate protocol, such as HTTP or HTTPS, to obtain the resource or its representation. A URI by itself is a name and not necessarily a locator; dereferencing is not guaranteed.

Examples: https://example.org/path/to/resource?query=value#section and urn:isbn:0451450523 illustrate URL and URN forms.