Every document on the Web has a unique address. (Imagine the chaos if they didn’t.) The document’s address is known as its uniform resource locator (URL).^[37]

Several HTML/XHTML tags include a URL attribute value, including hyperlinks, inline images, and forms. All use the same URL syntax to specify the location of a web resource, regardless of the type or content of that resource. That’s why it’s known as a uniform resource locator.

Since they can be used to represent almost any resource on the Internet, URLs come in a variety of flavors. All URLs, however, have the same top-level syntax:

            scheme:scheme_specific_part

The scheme describes the kind of object the URL references; the scheme_specific_part is, well, the part that is peculiar to the specific scheme. The important thing to note is that the scheme is always separated from the scheme_specific_part by a colon, with no intervening spaces.