Webmaster in a Nutshell, Third Edition

Uniform Resource Locators (URLs)

One of the most important things to grasp when working on the Web is the format for URLs. A URL is basically an address on the Web, identifying each document uniquely (for example, http://www.oreilly.com/products.html). Since URLs are so fundamental to the Web, we discuss them here in a little detail. The simple syntax for a URL is:

http://host/path

host

The host to connect to: www.oreilly.com or www.google.com. (While many web servers run on hosts beginning with www, the www prefix is just a convention.)

path

The document requested on that server. This is not the same as the filesystem path, as its root is defined by the server.

Most URLs follow this simple syntax. A more generalized syntax, however, is:

            scheme://host/path/extra-path-info?query-info

scheme

The protocol that connects to the site. For web sites, the scheme is http; for FTP, the scheme is ftp.

extra-path-info and query-info

Optional information used by CGI programs. See Chapter 12.

HTML documents also often use a “shorthand” for linking to other documents on the same server, called a relative URL. An example of a relative URL is images/webnut.gif. The browser knows to translate this into complete URL syntax before sending the request. For example, if http://www.oreilly.com/books/webnut.html contains a reference to images/webnut.gif, the browser reconstructs the relative URL as a full (or absolute) URL, http://www.oreilly.com/books/images/webnut.gif, and requests that document ...