HTML and CSS Reference
In-Depth Information
1.2.2. The Flow of Information
All web activity begins on the client side, when a user starts his browser.
The browser begins by loading a
home page
document, either from local
storage or from a server over some network, such as the Internet, a cor-
porate intranet, or a town extranet. When starting up on the network,
the client browser first consults a domain name system (DNS) server to
translate the home page server's name, such as
www.oreilly.com
, into
an IP address, before sending a request to that server over the Inter-
net. This request (and the server's reply) is formatted according to the
dictates of the
Hypertext Transfer Protocol
(HTTP) standard.
A server spends most of its time listening to the network, waiting
for document requests with the server's unique address stamped on
them. Upon receipt of a request, the server verifies that the requesting
browser is allowed to retrieve documents from the server and, if so,
checks for the requested document. If it finds the document, the server
sends it to the browser. The server usually logs the request, typically in-
cluding the client computer's IP address, the document requested, and
the time. The server might also issue special attachments known as
cookies
that contain additional information about the requesting browser
and its owner.
Back on the browser, the document arrives. If it's a plain-vanilla text
file, most browsers display it in a common, plain-vanilla way. Document
directories, too, are treated like plain documents, which most graphical
browsers display as folder icons that the user may select, thereby re-
questing to view the contents of the subdirectory.
Browsers can retrieve many different types of files from a server. Unless
assisted by a
helper
program or specially enabled by
plug-in
software or
applets
, which display an image or video file or play an audio file, the
browser usually stores the file directly on a local disk for later use.
For the most part, however, the browser retrieves a special document
that appears to be a plain text file but that contains both text and special
markup codes called
tags
. The browser processes these HTML or XHTML
documents, formatting the text based on the tags and downloading spe-
cial accessory files, such as images.
