Uniform Resource Identifier (URI)

A Uniform Resource Identifier (URI) is a sequence of characters used to identify a resource. Resources are distinguished by their location on the internet, their name, or both.

There are two types of URI: the Uniform Resource Locator (URL) and Uniform Resource Name (URN).

Table of Contents

URLs and URNs

A URI identifies physical or logical resources available on the web. Resources include web pages and electronic documents, but also people, places, objects, and abstract concepts.

A URI specifying the location of a resource is a Uniform Resource Locator (URL). A URI providing a persistent name without location details is a Uniform Resource Name (URN).

Every URL is a URI, and every URN is a URI. The terms overlap because URI is the umbrella category.

Syntax

The generic URI syntax follows this structure:

scheme ":" ["//" authority] path ["?" query] ["#" fragment]

Scheme

A required component naming the protocol or naming system. The scheme is case-insensitive, begins with a letter, and continues with any mix of letters, digits, plus +, hyphen -, or period .. Common schemes include http, https, ftp, mailto, and urn. IANA maintains the official URI schemes registry.

Authority

An optional component prefixed by two slashes //. The authority contains up to three sub-components:

  • Userinfo: optional username (and deprecated password), followed by @. The username:password form is deprecated for security reasons.
  • Host: a registered domain name or IP address. IPv6 addresses are enclosed in brackets [ ].
  • Port: optional, preceded by a colon :.

Path

A required component made up of segments separated by /. The path is required for every URI but its value is allowed to be empty (zero-length). A path following an authority is either empty or starts with /.

Query

An optional component prefixed by ?. The query string passes data or instructions to the resource, using a format expected by the server.

Fragment

An optional component prefixed by #. The fragment identifies a secondary resource or a specific part of the primary resource. Fragments are resolved on the client side and are never sent to the server in an HTTP request.

Percent-encoding

Characters outside the allowed set are represented using Percent-Encoding. Each encoded character uses a percent sign % followed by two hexadecimal digits representing the octet value. Reserved characters with special meaning in the URI syntax (:, /, ?, #, [, ], @, !, $, &, ', (, ), *, +, ,, ;, =) are percent-encoded when they appear in a component where they have no reserved purpose.

Relative references

A URI reference is either a full URI or a relative reference. Relative references omit the scheme and resolve against a base URI to produce a target URI. This allows document trees to remain portable across different locations and access schemes.

../images/logo.png

Given a base URI of https://example.re/docs/guide/index.html, the relative reference above resolves to https://example.re/docs/images/logo.png.

Examples

A full HTTP URI with scheme, authority, path, and fragment:

Example 1

http://www.example.re/doc/glossary#introduction
Component Value
Scheme http
Authority www.example.re
Path /doc/glossary
Fragment introduction

An HTTPS URI with port, path, query, and fragment:

Example 2

https://example.re:5001/scripts/?job=111&task=1#main
Component Value
Scheme https
Authority example.re:5001
Path /scripts/
Query job=111&task=1
Fragment main

A telephone URI with no authority. The tel scheme uses only a scheme and path:

Example 3

tel:+1-212-555-1212
Component Value
Scheme tel
Path +1-212-555-1212

An FTP URI pointing to a specific file:

Example 4

ftp://ftp.example.re/doc/glossary.pdf
Component Value
Scheme ftp
Authority ftp.example.re
Path /doc/glossary.pdf

See also

Last updated: August 11, 2026