Understanding URI, URL, and URN: Concepts, Structure, and Usage

When we open a browser to visit a website or an FTP server, we type a string such as https://blog.csdn.net/ or ftp://192.168.0.111/ , which yields an HTML page or a file; the meaning of this address is explained through URI, URL, and URN.

1. URI, URL, URN Concepts

URI = Uniform Resource Identifier

URL = Uniform Resource Locator

URN = Uniform Resource Name

A URI is an abstract identifier that can locate a resource by any method; a URL locates a resource by address, while a URN locates it by name. For example, finding a person in a village by address (URL) or by ID card and name (URN) both refer to the same underlying URI.

Originally, URI was envisioned to have two concrete forms: URL for address‑based location and URN for name‑based location. In practice, URNs are rarely used, so most URIs you encounter are URLs, making URL a subset of URI.

URL Definition

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

Examples:

Accessing a web page via HTTP:

Downloading/uploading files via FTP:

Reading a local file on the client:

Protocol

The first part of a URL is the protocol (e.g., http, https, ftp, mailto). It indicates which method the browser should use to access the resource and is followed by "//".

Username/Password

Credentials can be included but are often omitted.

Domain Name

The domain (e.g., www.gitee.com ) is resolved to an IP address via DNS unless the IP is used directly.

Port

An optional port follows the domain, separated by a colon. Default ports are 80 for HTTP, 443 for HTTPS, and 21 for FTP.

Path / Filename

The segment after the first "/" up to the last "/" is the virtual directory; the part after the last "/" up to "?" or "#" is the filename. If the filename is omitted, the server serves a default file such as /index.html or /default.htm .

Example of a filename omission:

http://www.gitee.com/dir/ – the trailing slash implies a missing filename, so the server returns the default file.

http://www.gitee.com – no path at all, the server returns the root default file.

http://www.gitee.com/yikoupeng – the server treats "yikoupeng" as a file if it exists, otherwise as a directory.

RFC References

For detailed specifications, see RFC 3986 for URIs and the URL RFC (e.g., https://www.rfc-editor.org/). The following excerpt shows part of the URL specification:

Network Working Group                     T. Berners-Lee
Request for Comments: 1738                CERN
Category: Standards Track                 L. Masinter

Uniform Resource Locators (URL)

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

Additional images illustrate how to browse the RFC repository.