Transcript IT 141: Information Systems I

Tonga Institute of Higher Education
IT 141: Information Systems
Lecture 10:
The World Wide Web
Outline
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In this chapter, you will learn about:
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The World Wide Web
HTML
HTTP and HTTPS protocols
Web browsers and servers
Cookies
Scripting languages and Java applets
Security
The World Wide Web
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In 1991, Tim Berners-Lee created a
protocol to provide a way for documents
to be viewed over a network
This was the beginning of the World Wide
Web
The WWW is not the same as the
Internet. The WWW is a popular service
that is offered over the network of
computers called the Internet
The Internet is a collection of wires and
computers connected together that send
many types of data between each other
Hypertext
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The advantage of viewing documents on
the computer allowed for the introduction
of hypertext
Hypertext is a way to turn normal text
into links that allow the user to click
through to other documents. The other
documents may contain more
information. This was an evolution in the
history of the written word.
Anytime you click on a link in a web
browser, you are using hypertext to
connect to another place
HTML
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Hypertext Markup Language is the
programming language used to create
websites
It adds in extra data to a normal text file
that changes the formatting of the
document.
A web server will store one or more
webpages, which make up a website.
Each webpage is an HTML document that
is stored on the server as a file.
HTTP
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When you request a webpage from a
server, your web browser uses a protocol
called HTTP to transfer the webpage.
HTTP – Hyper Text Transfer Protocol.
You will notice all the websites in your
browser start with this.
It is a protocol that uses TCP/IP to
transfer HTML pages
HTTP
HTML – Introduction
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HTML is called a markup language
because people can insert special
commands into documents to change the
way the documents appear
These special instructions are called HTML
tags
Tags are commands enclosed by brackets,
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Examples <BR> <HR> <B> <I>
You will not see the tag on your web
browser, but instead you will see what it
changes
HTML
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HTML tags usually work in pairs.
This means that if you want something to
be BOLD, you would put
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<b>Word to make bold</b>
You start a tag with the brackets and the
command
You end the tag with a ‘/’ before the command
These types of tags will modify the way
the web page appears
If you want the modification to end you
must have the end tag.
HTML documents
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HTML documents look different from
webpages.
A web browser will “render” an HTML
document into what you see on the web
browser.
This “rendering” is like compiling, except
you can still see the source code by rightclicking on a page and going to “View
Source”
Web Browsers
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Your browser is what lets you see a
webpage.
It will do two things:
1. Send a HTTP request for a webpage
2. Render an HTML document
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The first browser was called Mosaic, made
in 1993
Today there are many good browsers
available. Examples:
1. Internet Explorer (90% market share)
2. Mozilla/Firefox (8% market share)
3. Opera (1% market share)
How does HTTP work
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HTTP is a TCP/IP protocol because you need to establish a
connection with the web server and your computer before
data is successfully exchanged.
HTTP uses simple commands to request information and also
to send information back to a web server
One common command is “GET”, and it allows web browsers
to retrieve a web page from a server
To test this, you can connect to a webserver without your
browser and then try to "speak" the HTTP protocol
On Windows computers, go to Start -> Run and type
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A connection will be established with the web server and then
type
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telnet www.tihe.org 80
GET /
This command will get the main webpage of the www.tihe.org
What do you see when you type this?
Web Servers
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A web server is a program that waits for
HTTP connections to be made
When a connection is made it reads the
HTTP request to see what is wanted and
then sends back the correct file
Web servers can do many requests at the
same time, but for really big web sites,
like yahoo.com, they need many
hundreds of computer to all act as server
Is a web server the same as a website?
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No! Sometimes a web server will have more
than one website on it
Cookies
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After the first few years of using HTTP, people
realized the need for websites to be able to save
information on the user's computer and then later
retrieve it.
For example: to remember passwords, help with
user authentication, save shopping carts, etc.
Cookies are small files that are saved on your
computer by websites.
They allow websites to "remember" you over a
period of time
To view the cookies for Internet Explorer, go to
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C:\Documents and Settings\<username>\Cookies
Cookies
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Whenever data is exchanged over a network, there is a
possibility that one end of the connection may try to
send or get data that they are not meant to get
Cookies could potentially be dangerous to a user if, for
example, your username and password for your bank
account was saved in a cookie and then a webserver
owned by a hacker could read that username and
password
Web browsers are responsible for limiting who is
allowed to access cookies.
A website cannot read cookies that it did not make
If yahoo.com leaves a cookie on your computer,
microsoft.com cannot read it or use it even if you go to
the website microsoft.com
Some cookies on your computer forever, some will
delete after you close the browser. It depends on what
the website does.
Beyond HTML
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HTML is good for formatting webpages, but many
people wanted more power to display and design
webpages.
DHTML – Dynamic HTML – A language that allows
extra functionality in HTML.
 It is not a replacement for HTML, but an
addition, to make HTML do more exciting things
Flash – a way to create animated displays with
sound in a way that does not create very large
files. (Large files take a long time to download on
the Internet).
Javascript and VBScript
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Scripts are ways to add programming to a webpage.
HTML is sort of like a programming language, but
differs in some important ways.
HTML is a Markup Language
JavaScript and VBScript are programming languages
that allow logic to be inserted into the way a
webpage operates
For example, they are often used to make sure you
are entering correct information into forms.
If you go to the Web Email on www.tihe.org, and
press Login without typing a password, a message
will appear that tells you do type in a password. This
is JavaScript in action.
Client Side Scripts
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Javascript and VBScript are called client-side
scripts, because the clients computer (or web
browser) runs the program.
Your web browser downloads the source code
for these programs (often they are included
in the HTML file)
Your browser is then responsible for
compiling the code and executing it within
the web browser
Server Side Scripts
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Server-side scripts are programs that run on the server,
instead of the client.
A web browser will request a webpage and the webserver
may notice that there is programming within the
webpage.
The webserver will compile and execute the code and then
send an HTML page back to the user.
The web browser never even seen the source code of the
scripts, unlike client-side scripts.
WebEmail is an example of server side scripting.
Notice, when you login the email displayed is different
from another person’s email.
There is a script on the server that changes what email is
displayed based on the username and password entered
Some scripting languages: PHP, Perl, ASP or ColdFusion.
(The web email at tihe.org uses PHP)
Java applets
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Java is a real programming language and
it needs to be compiled before you can
use it.
Scripts are different in that you can
execute a script without compiling before.
Java applets are Java programs made for
webpages.
They are a useful way for adding
programs that may display graphics and
request user input at the same time
The way Java applets work
Security on the Internet
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When you make a request for a webpage over the
internet, if it is uses the HTTP protocol, it is called
unsafe or unsecure.
It is unsecure because the data is sent
unencrypted (or in plain text) over the Internet
For most webpages this is not important, but if the
web page you go to asks for your username and
password, you don't want other people to see it
How do other people see it? When your data
travels over the Internet it must go through many
different computers. Each computer it goes
through provides an opportunity for the
person/company who owns the computer to look at
the data
Security on the Internet
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There is a secure protocol called HTTPS
(the extra S is for secure).
This encrypts data between you and the
server
Encryption is a way to turn normal text
into a form that is almost impossible to
turn back unless you know the secret
password.
If you enter important information about
yourself on a webpage, make sure it says
HTTPS in the URL (the place where you
type the name of the website)
Summary
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The World Wide Web (WWW)
HTML
HTTP and HTTPS protocols
Web browsers and servers
Cookies
Scripting languages and Java
applets
Security