World Wide Web 1

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World Wide Web 1

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Networks allow computers to talk to each other

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Networks can have different topologies

Linear

Star

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If a node on a network fails other nodes might be disconnected

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The Internet

• During the Cold War the U.S. Military gave money to colleges and universities to develop networks that could survive a nuclear attack

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A network with a center would be very vulnerable to attach

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A distributed network with multiple routes between points would be

more likey to survive

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The network relied on the ability of the nodes to dynamically find

routes for data

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The Internet

• The result was a global decentralized network that no one owns.

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Network Protocols• E-mail: allows users to

send messages to each other

• FTP: File Transfer Protocol

• Telnet: allows users to log into their computers from remote locations

• A number of network protocols were developed for the transfer of information

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Hyper Text Transfer Protocol

• HTTP allowed information for public consumption to be transferred and read easily

• HyperText Markup Language was developed for describing the content

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What happens when you look at a web page?

• Browser• Link• Network• Server• HTML file• Network• Image file• Browser

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Browser

• Web pages are displayed in a browser

• Common Browsers– Internet Explorer (MS)

– Navigator (Netscape)

– Opera

– Safari (Apple)

– Firefox

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Link

• In the browser the user can– Click on a link in the

current page

– Type in the URL for a new page e.g www.cit.ie

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Network

• The URL request for data is routed through the internet until it finds its way to the web server

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Server

• The web server accepts requests from web surfers for files it holds.

• Using the URL it determines who wants what and then goes away and gets it

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Network

• The file the web surfer requested is sent across the network to the surfer’s browser

• When it arrives the browser interprets the HTML and lays out the page according to instructions

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Image File

• If the web page the user requested contains images then the browser sends a request to the server to the images

• When these images arrive the browser can insert them into the page

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Browser

• The browser lays out the web page with images as specified by the HTML

• Different browsers might render the HTML slightly differently

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Broken Link

• If the surfer requests a file that is not on the server then the web server cannot deliver it

• This happens with mis-typed URLs

• If a link points to a file that is not available then is is a broken link

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HTML Files

• HTML files are bog-standard unformatted text files

• By convention HTML files on the web server are named with extensions .htm or .html

• The URL must specify the server name and the precise location of the file in the directory system

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Images

• While modern browsers can render a variety of image formats only GIF and JPEG are considered standard

• GIF Graphics Interchange Format

• JPEG Joint Photographic Experts Group

• PNG

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H.T.M.L.

• Web pages are written Hypertext Mark up Language

• HTML pages are ASCII text files with tags for formatting

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HTML Example: 1 Basic Page

• Tags are bounded by < and >

• Tags usually occur in pairs

• All of the content appears in the body

<html><head></head><body>Hello World</body></html>

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HTML Example 2: Bold & Italics

Hello World

Howdy Doody

<html><head></head><body>Hello World HowdyDoody</body></html>

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HTML Example 3: A Link

Hello World

Howdy Doody

CIT

<html><head></head><body>Hello World HowdyDoody <a

href=“http://www.cit.ie/”>CIT</a>

</body></html>

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More Examples on Handouts

• • <hr/>• <a href=“”>• <img src=“”/>• <tt><code>

World Wide Web 1

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