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In the field of computer networking, a URI scheme is the top level of the uniform resource identifier (URI) naming structure. All URIs and absolute URI references are formed with a scheme name, followed by a colon character (":"), and the remainder of the URI called (in the outdated RFCs 1738 and 2396, but not the current STD 66/RFC 3986) the scheme-specific part. The syntax and semantics of the scheme-specific part are left largely to the specifications governing individual schemes, subject to certain constraints such as reserved characters and how to "escape" them.

URI schemes are frequently erroneously referred to as "protocols", or specifically as URI protocols or URL protocols, since most were originally designed to be used with a particular protocol, and often have the same name. The http scheme, for instance, is generally used for interacting with Web resources using HyperText Transfer Protocol. Today, URIs with that scheme are also used for other purposes, such as RDF resource identifiers and XML namespaces, that are not related to the protocol. Furthermore, some URI schemes are not associated with any specific protocol (e.g. "file") and many others do not use the name of a protocol as their prefix (e.g. "news").

URI schemes should be registered with IANA, although non-registered schemes are used in practice. RFC 4395 describes the procedures for registering new URI schemes.

Generic syntax[link]

Internet standard STD 66 (also RFC 3986) defines the generic syntax to be used in all URI schemes. Every URI is defined as consisting of four parts, as follows:

<scheme name> : <hierarchical part> [ ? <query> ] [ # <fragment> ]

The scheme name consist of a sequence of characters beginning with a letter and followed by any combination of letters, digits, plus ("+"), period ("."), or hyphen ("-"). Although schemes are case-insensitive, the canonical form is lowercase and documents that specify schemes must do so with lowercase letters. It is followed by a colon (":").

The hierarchical part of the URI is intended to hold identification information hierarchical in nature. Usually this part begins with a double forward slash ("//"), followed by an authority part and an optional path.

• The authority part holds an optional user-information part, terminated with "@" (e.g. username:password@); a hostname (e.g., domain name or IP address); and an optional port number, preceded by a colon ":".

• The path part is a sequence of segments (conceptually similar to directories, though not necessarily representing them) separated by a forward slash ("/"). Each segment can contain parameters separated from it using a semicolon (";"), though this is rarely used in practice.

The query is an optional part, separated by a question mark ("?"), that contains additional identification information that is not hierarchical in nature. The query string syntax is not generically defined, but it is commonly organized as a sequence of <key>=<value> pairs, with the pairs separated by a semicolon^[1][2][3] or an ampersand. For example:

Semicolon: key1=value1;key2=value2;key3=value3
Ampersand: key1=value1&key2=value2&key3=value3

The fragment is an optional part separated from the front parts by a hash ("#"). It holds additional identifying information that provides direction to a secondary resource, e.g., a section heading (in an article) identified by the remainder of the URI. When the primary resource is an HTML document, the fragment is often an id attribute of a specific element and web browsers will make sure this element is visible.

Examples[link]

The following figure displays two example URIs (foo://username:password@example.com:8042/over/there/index.dtb?type=animal&name=narwhal#nose and urn:example:animal:ferret:nose) and their component parts. (The examples are derived from RFC 3986 — STD 66, chapter 3).

  foo://username:password@example.com:8042/over/there/index.dtb?type=animal&name=narwhal#nose
  \_/   \_______________/ \_________/ \__/            \___/ \_/ \______________________/ \__/
   |           |               |       |                |    |            |                |
   |       userinfo         hostname  port              |    |          query          fragment
   |    \________________________________/\_____________|____|/ \__/         \__/ 
   |                    |                          |    |    |    |           |         
scheme              authority                    path   |    |    interpretable as keys
 name   \_______________________________________________|____|/       \____/     \_____/ 
   |                         |                          |    |          |           |
   |                 hierarchical part                  |    |    interpretable as values
   |                                                    |    |
   |            path               interpretable as filename |
   |   ___________|____________                              |
  / \ /                        \                             |
  urn:example:animal:ferret:nose               interpretable as extension

 scheme
  name  userinfo  hostname       query
  _|__   ___|__   ____|____   _____|_____
 /    \ /      \ /         \ /           \
 mailto:username@example.com?subject=Topic

Official IANA-registered schemes[link]

The official URI schemes registered with the IANA are as follows (note that "IETF Draft" in no way is a finalized specification and must not be treated so; refer to Section 2.2 of RFC 2026 for more details):

Unofficial but common URI schemes[link]

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