Post on 17-Jan-2016
description
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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<br/><b>Howdy</
b><i>Doody</i></body></html>
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HTML Example 3: A Link
Hello World
Howdy Doody
CIT
<html><head></head><body>Hello World<br/><b>Howdy</b><i>Doody</
i><br/><a
href=“http://www.cit.ie/”>CIT</a>
</body></html>
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More Examples on Handouts
• <b><i><u>• <br/> <hr/>• <a href=“”>• <img src=“”/>• <tt><code>