URIs

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 optional. A more complex example might look like this:

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

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.

We will break down the following URL into its components:

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

http:// is the scheme of the URL, indicating 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 a file transfer, so don't be surprised if you see such protocols. The schemes reference provides a list of the most common schemes, and documentation for some of them.

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.

The URI authority is comprised of user info (optional and usually unspecified), the host name, and the port.

www.example.com is the host name of the URI, indicating which Web server is being requested. Here, we use a domain name. It is also possible to directly use an IP address, but because it is less convenient, it is rare to do so, unless the server doesn't have a registered domain name.

:80 is the port of the URL, indicating 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/to/myfile.html is the path of the URL, indicating the location of the resource on the web server. In the early days of the Web, this was an actual directory path to a physical location on the web server. Nowadays, web servers usually abstract this to an arbitrary location.

?key1=value1&key2=value2 is the query of the URL, which are extra parameters provided to the web server. The parameters are a list of key/value pairs prefixed by the ? symbol, and separated with the & symbol. These can be used to provide additional context about the resource being requested. Each resource location can have its own rules regarding parameters, and the only reliable way to know how specific parameters are handled is by asking the owner of the server, such as reading its documentation.

#SomewhereInTheDocument is the fragment of the URL, which 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 the fragment identifier, is never sent to the server with the request.

There's a special text fragment feature that allows you to link to a specific part of a web page identified by its text content.

• What is a URL?