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page 85

way for an outside computer to access a computer behind the firewall.

4.4 FORMATS

Formats are different from protocols, although they are often confused. Protocols are used for transferring information, but formats define the information format. For example, http is a protocol often used for transferring files. The most common file format transferred with http is html, although it can transfer other types of files.

4.4.1 HTML

Hyper Text Markup Language (HTML) is the defacto standard format for information on the World Wide Web (WWW). It basically allows text to be generally formatted with embedded images. When viewed, the display is adjusted to suit the browser. These documents also include hypertext links that allow a user to go to another HTML page, or download a file by ’clicking’ on linked text. Recently there have been many additions that allow more control over the ’look-and- feel’, but result in larger files and less portability. HTML files can be created with programs, or edited by hand.

A simple HTML file is shown in Figure X.X. It uses tags to define the beginning and end of definitions. The entire document begins and ends with the tags ’<HTML>’ and ’</HTML>’. The body of the document begins and ends with ’<BODY>’ and ’</BODY>’. Highlighted text is between ’<Hx>’ and ’</Hx>’, where ’x’ varies from ’1’ for the boldest to ’5’ to the lightest. Lists can be defined with the tags ’<UL>’ and ’</UL>’, and each point in the list begins with ’<LI>’.

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HTML File:

<HTML>

 

<BODY>

<H1>Title</H1> <P>This is a list</P> <UL>

<LI>One

<LI>Two

</UL>

</BODY>

</HTML>

The Output:

Title

This is a list

One

Two

Figure X.X - A Simple HTML File and The Output

The file in Figure X.X add a few features to the previous example. The first part is a header section that defines a title of the document. This title will appear on the top bar of the browser window. An image will appear after the heading and before the list, the image displayed is called ’test.gif’. The two items in the list now have hypertext links. The first is to a file called ’other.html’, the second is to another web site ’www.cnn.com’.

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<HTML>

<HEAD>

<TITLE>A Sample Page</TITLE>

</HEAD>

<BODY>

<H1>Title</H1> <P>This is a list</P> <IMG SRC="test.gif"> <UL>

<LI><A HREF="another.html">One</A>

<LI><A HREF="http://www.cnn.com">Two</A>

</UL>

</BODY>

</HTML>

Figure X.X - A More Advanced HTML File

4.4.2 URLs

In HTML documents we need to refer to resources. To do this we use a label to identify the type of resource, followed by a location.

Universal Resource Locators (URLs)

-http:WEB_SITE_NAME

-ftp:FTP_SITE_NAME

-mailto:USER@MAIL_SERVER

-news:NEWSGROUP_NAME

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4.4.3 Encryption

Allows some degree of privacy, but this is not guaranteed.

Basically, if you have something you don’t want seen, don’t do it on the computer.

4.4.4 Clients and Servers

Some computers are set up to serve others as centers of activity, sort of like a campus library. Other computers are set up only as users, like bookshelves in a closed office. The server is open to all, while the private bookshelf has very limited access.

A computer server will answer requests from other computers. These requests may be,

-to get/put files with FTP

-to send email

-to provide web pages

A client does not answer requests.

Both clients and servers can generate requests.

Any computer that is connected to the network Client or Server must be able to generate requests. You can see this as the Servers have more capabilities than the Clients.

Microsoft and Apple computers have limited server capabilities, while unix and other computer types generally have more.

Windows 3.1 - No client or server support without special software Windows 95 - No server support without special software Windows NT - Limited server support with special versions MacOS - Some server support with special software

Unix - Both client and server models built in

In general you are best advised to use the main campus servers. But in some cases the extra effort to set up and maintain your own server may also be useful.

To set up your own server machine you might,

1.Purchase a computer and network card. A Pentium class machine will actually provide more than enough power for a small web site.

2.Purchase of copy of Windows NT server version.

3.Choose a name for your computer that is easy to remember. An example is ‘artsite’.