Understanding URI, URL, and URN: The Core of Web Resource Identification

URI, URL, URN Concepts

URI = Uniform Resource Identifier

URL = Uniform Resource Locator

URN = Uniform Resource Name

URI is an abstract identifier that can be expressed in various ways; if it can locate a resource, it is a URI. Historically two concrete methods were envisioned: URL for address‑based location and URN for name‑based location.

In practice URNs never gained wide adoption, so most URIs you encounter are URLs. The diagram below (originally from the source) shows the geometric relationship, confirming that a URL is a subset of a URI.

What a URL Is

A URL (Uniform Resource Locator) tells a client where a resource resides on the Web. The resource may be an HTML page, a CSS file, an image, a video, etc.

Protocol

The URL always begins with a scheme such as http, https, ftp, or mailto. The scheme determines which protocol the browser should use.

Username/Password

Credentials can be embedded (e.g., user:pass@) but are optional and often omitted.

Domain

The domain (e.g., www.gitee.com) is resolved via DNS to an IP address; if the IP is known, DNS lookup can be skipped.

Port

A port follows the domain after a colon ( :). It is optional; default ports are 80 for HTTP, 443 for HTTPS, and 21 for FTP.

Path and File Name

The path starts with the first slash after the domain and continues to the last slash before the file name. The file name may be followed by a query ( ?) or fragment ( #). If the file name is omitted, the server supplies a default file such as index.html or default.htm.

http://www.gitee.com/dir/        # trailing slash → directory, default file used
http://www.gitee.com            # no trailing slash → root directory, default file used
http://www.gitee.com/yikoupeng   # could be a file or a directory depending on server

When a directory path ends with a slash, the server looks for a default file (commonly index.html or default.htm) and serves it.

RFC Reference

The formal specifications for URLs are defined in RFC 3986 and related documents. They can be consulted at the RFC Editor site: https://www.rfc-editor.org/ Below is an excerpt from the RFC that defines the syntax and semantics of URLs.

Uniform Resource Locators (URL)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.

Abstract

   This document specifies a Uniform Resource Locator (URL), the syntax
   and semantics of formalized information for location and access of
   resources via the Internet.

The article concludes that understanding the structure of URIs, URLs, and URNs is essential for working with web resources, debugging network issues, and designing interoperable systems.