A URL, or Uniform Resource Locator, is a structured identifier interpreted according to a URL scheme and parsing rules. On the web it can identify the resource a browser navigates to or requests. The URL parser produces a record with components such as scheme, host, port, path, query and fragment; which components are present depends on the particular URL.[ref]
For example,
https://example.org:8443/a/b?x=1#part has scheme https, host example.org, port 8443, path /a/b, query string x=1 and fragment identifier part. This hierarchical HTTPS example is useful without implying that every URL must contain a network host or an explicit port.A Relative URL reference needs a base. Resolving
../img/p.png against https://example.org/a/b/page.html gives https://example.org/a/img/p.png. The base's directory and parent-directory step explain the result. Parsing and serialization can also normalize a spelling: https://EXAMPLE.org:443/a serializes as https://example.org/a, lowercasing the host and omitting the default HTTPS port.[ref]Percent-encoding represents bytes in a URL spelling, and encoded data must be distinguished from structural delimiters. A query string is not universally a dictionary of name/value pairs, even though many applications use that convention. Nor does successful parsing prove that a resource is reachable or that its content never changes. Distinguishing syntax, identifier interpretation and an actual retrieval prevents an address from being confused with the information returned on one occasion.
