Identifying resources on the Web

The target of an HTTP request is called a "resource", which nature isn't defined further; it can be a document, a photo, or anything else. Each resource is identified by a Uniform Resource Identifier (URI) used throughout HTTP for identifying resources.

The identity and the location of resources on the Web are mostly given by a single URL (Uniform Resource Locator, a kind of URI). There are sometimes reasons identity and location are not given by the same URI: HTTP uses a specific HTTP header, Alt-Svc when the resource requested wants the client to access it at another location.

URLs and URNs

URLs

The most common form of URI is the Uniform Resource Locator (URL), which is known as the web address.

https://developer.mozilla.org
https://developer.mozilla.org/en-US/docs/Learn/
https://developer.mozilla.org/en-US/search?q=URL

Any of those URLs can be typed into your browser's address bar to tell it to load the associated page (resource).

A URL is composed of different parts, some mandatory and others are optional. A more complex example might look like this:

http://www.example.com:80/path/to/myfile.html?key1=value1&key2=value2#SomewhereInTheDocument

URNs

A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace.

urn:isbn:9780141036144
urn:ietf:rfc:7230

The two URNs correspond to

• the book Nineteen Eighty-Four by George Orwell,
• the IETF specification 7230, Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing.

Syntax of Uniform Resource Identifiers (URIs)

Scheme or protocol

http:// is the protocol. It indicates which protocol the browser must use. Usually it is the HTTP protocol or its secured version, HTTPS. The Web requires one of these two, but browsers also know how to handle other protocols such as mailto: (to open a mail client) or ftp: to handle file transfer, so don't be surprised if you see such protocols. Common schemes are:

Authority

www.example.com is the domain name or authority that governs the namespace. It indicates which Web server is being requested. Alternatively, it is possible to directly use an IP address, but because it is less convenient, it is not often used on the Web.

Port

:80 is the port in this instance. It indicates the technical "gate" used to access the resources on the web server. It is usually omitted if the web server uses the standard ports of the HTTP protocol (80 for HTTP and 443 for HTTPS) to grant access to its resources. Otherwise it is mandatory.

Path

/path/to/myfile.html is the path to the resource on the Web server. In the early days of the Web, a path like this represented a physical file location on the Web server. Nowadays, it is mostly an abstraction handled by Web servers without any physical reality.

Query

?key1=value1&key2=value2 are extra parameters provided to the Web server. Those parameters are a list of key/value pairs separated with the & symbol. The Web server can use those parameters to do extra stuff before returning the resource to the user. Each Web server has its own rules regarding parameters, and the only reliable way to know how a specific Web server is handling parameters is by asking the Web server owner.

Fragment

#SomewhereInTheDocument is an anchor to another part of the resource itself. An anchor represents a sort of "bookmark" inside the resource, giving the browser the directions to show the content located at that "bookmarked" spot. On an HTML document, for example, the browser will scroll to the point where the anchor is defined; on a video or audio document, the browser will try to go to the time the anchor represents. It is worth noting that the part after the #, also known as fragment identifier, is never sent to the server with the request.

Usage notes

When using URLs in HTML content, you should generally only use a few of these URL schemes. When referring to subresources — that is, files that are being loaded as part of a larger document — you should only use the HTTP and HTTPS schemes. Increasingly, browsers are removing support for using FTP to load subresources, for security reasons.

FTP is still acceptable at the top level (such as typed directly into the browser's URL bar, or the target of a link), although some browsers may delegate loading FTP content to another application.

Examples

https://developer.mozilla.org/en-US/docs/Learn
tel:+1-816-555-1212
[email protected]:mdn/browser-compat-data.git
ftp://example.org/resource.txt
urn:isbn:9780141036144
mailto:[email protected]

Specifications

See also

• What is a URL?
• IANA list of URI schemes