Every resource available on the Web — HTML document, image, video clip, program, etc. — has an address that is specified as a Universal Resource Identifier, or URI.

URIs typically consist of three parts:

• the naming scheme of the mechanism used to access the resource.
• the name of the machine hosting the resource.
• the name of the resource itself, given as a path.

To access hypertext, we write the resource identifier as:

http://hostname/path/filename

For instance, to point to the W3C documentation on HTML 4.0, we would write:

http://www.w3.org/TR/REC-html40/

which means that there is a document available via the HTTP protocol, residing on the machine www.w3.org, and accessible via the path /TR/REC-html40/.

Other URI schemes that are used in HTML documents include ftp: for File Transfer Protocol and mailto: for email.

For file transfers, or downloads, we write:

ftp://hostname/path/filename

To send email, we write the address a little differently:

mailto:someone@hostname

Many readers may be familiar with the term URL, for Uniform Resource Locator. URLs form a subset of the general URI naming scheme.