How does the Internet work? - The Tao of Computing - page 310

Table 11.2 Some Common File Name Endings and the File Types
They Often Denote

File Name Ending In Format Used in Document

.hmtl or .htm Hypertext Markup Language

.pdf Portable Document Format

.ps PostScript

.gif or .jpeg graphical GIF or JPEG format, as discussed in Chapter 2

As this suggests, a document with the full name
http://www.cs.grinnell.edu/~walker/index.html
would be located on the Web server www.cs.grinnell.edu in the ac
count of user walker. The file itself is called index.html. The com
puter uses the suffix html or htm to know the format used in the
document. As with much material found on the World Wide Web,
this file contains formatting instructions based on a system called
the Hypertext Markup Language (HTML) to specify how the docu
ment should look. (We will talk more about HTML shortly.)
As a second, more formal, approach for communication of types
of documents, a Web server may precede its sending of a file with a
specific message, notifying the browser about the nature of the doc
ument. In this case, a server may indicate the forthcoming document
will be in PDF, PostScript, or another format, regardless of the suf
fix used in the document name. In such situations, a browser is sup
posed to interpret what follows in the designated way (although oc
casionally a browser may not function properly).
Once a browser knows or infers the format of material it is receiv
ing, it interprets the document using one of two mechanisms. If the for
mat of the document is common (like HTML), the browser may know
how to process it and can display the document directly. Alternatively,
the browser may consult a table of formats and run a designated, special
program to carry out the actual display. Such a designated program is
sometimes called a plugin . Through the use of plugins, a browser can re
spond to a wide range of materials, and new formats can be added with
out changing the browser. (Only the table of plugins need be updated.)

How does HTML work?

HTML is P reasonably streamlined approach for specifying
formats and display information for Web documents. To be