HTTP: The Definitive Guide

Navigating the Internet’s Resources

URLs are the resource locations that your browser needs to find information. They let people and applications find, use, and share the billions of data resources on the Internet. URLs are the usual human access point to HTTP and other protocols: a person points a browser at a URL and, behind the scenes, the browser sends the appropriate protocol messages to get the resource that the person wants.

URLs actually are a subset of a more general class of resource identifier called a uniform resource identifier, or URI. URIs are a general concept comprised of two main subsets, URLs and URNs. URLs identify resources by describing where resources are located, whereas URNs (which we’ll cover later in this chapter) identify resources by name, regardless of where they currently reside.

The HTTP specification uses the more general concept of URIs as its resource identifiers; in practice, however, HTTP applications deal only with the URL subset of URIs. Throughout this book, we’ll sometimes refer to URIs and URLs interchangeably, but we’re almost always talking about URLs.

Say you want to fetch the URL http://www.joes-hardware.com/seasonal/index-fall.html:

• The first part of the URL (http) is the URL scheme . The scheme tells a web client how to access the resource. In this case, the URL says to use the HTTP protocol.

• The second part of the URL (www.joes-hardware.com) is the server location. This tells the web client where the resource is hosted.

• The third ...