A URI scheme is the top level of the URI naming structure. All absolute URIs are formed with a scheme name, followed by a colon character, and the remainder of the URI called 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 sometimes erroneously referred to as "protocols", or specifically as URI protocols or URL protocols, since most are associated with particular protocols, and often have the same name. The http: scheme, for instance, is used for referencing HTTP resources. However, 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:"), making the label "protocol" misleading.

URI schemes should be registered with IANA, although many non-registered schemes are used in practice. RFC 2717 describes the procedures for registering new URI schemes. RFC 2718 provides guidelines for the development of new URI schemes. Both of these RFCs are currently undergoing revision.

Generic syntax

---

RFC 2396, which defines the general syntax for URIs, also defines a "generic URI" syntax to be used by "hierarchical" schemes such as "http:". This is defined as consisting of four parts, as follows: ://?

The "authority" part is further broken down, for schemes representing IP-based protocols, into an optional part terminated with "@" (e.g. username:password@), a hostname (i.e. domain name or IP address), and an optional port number (see TCP and UDP ports) preceded by ":".

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

The part is not defined further generically, but is commonly used in http: URIs as a sequence of = pairs, separated by "&", e. g. key1=value1&key2=value2&key3=value3.

Thus a full example, for referencing an HTTP resource, would be: http://alice:secret@www.example.com:80/path/to/document?foo=bar In practice, the part is rarely used, and 80 is the default port for HTTP requests, so the following is more likely: http://www.example.com/path/to/document?foo=bar

List of URI schemes

---

Some URI schemes are listed here:

Scheme	Purpose	Defined by	General format	Notes	-	Official IANA-registered schemes					-	http:	HTTP resources	RFC 2616	generic syntax			https:	HTTP connections secured using SSL/TLS	RFC 2817	generic syntax			ftp:	FTP resources	RFC 1738	generic syntax			mailto:	SMTP e-mail messages and content	RFC 2368	mailto: ?header1=value1&header2=value2	Headers are optional, but often include "subject"; body= can be used to pre-fill the body of the message.		file:	addressing files on local or network file systems	RFC 1738	generic syntax (often appears as file:///path, the 3rd '/' representing the root directory of the local system)	Unusual in not being bound to any network protocol, and not usable in an Internet context.		urn:	Uniform Resource Names	RFC 2141	urn::			URI scheme	inclusion of small data items inline	RFC 2397	data:;base64,			news:	(Usenet) newsgroups and postings	RFC 1738	news: or news:	References a particular resource, regardless of location.		nntp:	Usenet NNTP	RFC 1738	nntp://://	Referencing a specific host is often less useful than referencing the resource generically, as NNTP servers are not always publically accessible		imap:	accessing mailbox through IMAP	RFC 2192				pop:	accessing mailbox through POP3	RFC 2384				pres:	Used in Common Profile for Presence to identify presentitis	RFC 3859	pres: ?header1=value1&header2=value2	Similar to "mailto:"		sip:	used with Session Initiation Protocol (SIP)	RFC 3969 RFC 3261				sips:	secure equivalent of sip:	RFC 3969 RFC 3261				ldap:	LDAP directory request	RFC 2255 RFC 4516				tel:	used for telephone numbers	RFC 3966	tel:			telnet:	used with telnet	RFC 4248				nfs:	Network File System resources	RFC 2224	generic syntax			wais:	used with Wide area information server (WAIS)	RFC 4156				gopher:	used with Gopher protocol	RFC 4266 ?	gopher://://			cid:	accessing body parts inside a message	RFC 2392	cid:	Used inside a message to refer to other parts of the same message.		mid:	accessing messages or parts of messages	RFC 2392	mid: or mid:/	Used to refer to messages or parts of a message	--	Unofficial but common URI schemes						URI scheme	Displaying product information and internal information	Un-standardised		Widely used by web browsers, sometimes even providing interactive resources		aim:	Launching AOL Instant Messenger conversations	AOL	aim:goim?screenname=&message=			callto:	Launching Skype call	Skype	callto:///	Works with current version of *" target="_blank" >and [Internet Explorer		skype:	Launching Skype call (official)	Skype	skype:?	Official documentation from Skype website		URI scheme	resources available using the eDonkey2000 network	eDonkey2000	ed2k:// >file	Links to servers are also possible, as are additional parameters.		URI scheme	web feed subscription		feed:*			lastfm:	Launching Lastfm music client	Lastfm	lastfm://globaltags/ or lastfm://user//			URI scheme	"magnet links"		magnet:?xt=urn:sha1:&dn= (other parameters are also possible)	Used by various peer-to-peer clients, usually providing the hash of a file to be located on the network.		xfire:	Adding friends and servers, joining servers, changing status text.	Xfire	xfire:?=&=	Official documentation from Xfire website		xmpp:	XMPP (Jabber)	Jabber SoftwareFoundation				Unofficial, unused, and unknown (not obsolete) URI schemes						netcmd:	Running NetCMD commands without telnet through the NetCMD Command Engine	The NetCMD Project	netcmd:			worldnet:	Accessing data on the WorldNet information network (Similar to http:)	The NetCMD Project	worldnet:////.$			Scheme	Purpose	Defined by	General format	Notes

External links

---

• Official IANA Registry of URI Schemes

URI scheme