Fragment identifier

In computer hypertext, a fragment identifier is a short string of characters that refers to a resource that is subordinate to another, primary resource. The primary resource is identified by a Uniform Resource Identifier (URI), and the fragment identifier points to the subordinate resource.

The fragment identifier introduced by a hash mark # is the optional last part of a URL for a document. It is typically used to identify a portion of that document. The generic syntax is specified in RFC 3986. The hash mark separator in URIs does not belong to the fragment identifier.

In URIs a hash mark # introduces the optional fragment near the end of the URL. The generic RFC 3986 syntax for URIs also allows an optional query part introduced by a question mark ?. In URIs with a query and a fragment, the fragment follows the query. Query parts depend on the URI scheme and are evaluated by the server — e.g., http: supports queries unlike ftp:. Fragments depend on the document MIME type and are evaluated by the client (Web browser). Clients are not supposed to send URI-fragments to servers when they retrieve a document, and without help from a local application (see below) fragments do not participate in redirections.

A URI ending with # is permitted by the generic syntax and could be considered as a kind of empty fragment. In MIME document types such as text/html or any XML type, empty identifiers to match this syntactically legal construct are not permitted. Web browsers typically display the top of the document for an empty fragment.

The fragment identifier functions differently than the rest of the URI: namely, its processing is exclusively client-side with no participation from the web server — of course the server typically helps to determine the MIME type, and the MIME type determines the processing of fragments. When an agent (such as a Web browser) requests a web resource from a Web server, the agent sends the URI to the server, but does not send the fragment. Instead, the agent waits for the server to send the resource, and then the agent processes the resource according to the document type and fragment value.

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