The Java Crypto API - Computer Science
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Transcript The Java Crypto API - Computer Science
DNS – HTTP – DHTML - CSS
ICW Lecture 5
Hasan Qunoo
DNS - Domain Name System
School of Computer Science
The University of Birmingham
Edgbaston
Birmingham
B15 2TT
United Kingdom
.uk
.ac
.bham
.cs
www
DNS - Domain Name System
• Everyone has used a DNS.
• The DNS system forms one of the largest and most
active distributed databases on the planet.
• a hierarchical, domain-based naming scheme and a
distributed database system for implementing this
naming scheme.
• Every machine on the Internet has its own IP address.
Some static and some dynamic.
• An IP address is all that you need to talk to a server.
• Domain names are strictly a human convenience.
DNS - Domain Name System
• The DNS system is a database
• It translates symbolic hostnames into the numerical
IP addresses.
Example:
www.cs.bham.ac.uk 147.188.192.42
www.bham.ac.uk
147.188.125.57
www.google.com
74.125.43.105
• A single URL can be resolved into more than one
IP. (Why?)
DNS- How does it work?
When a request comes in:
• DNS can answer the request with an IP address because it already knows
the IP address for the domain.
• DNS can contact another name server and try to find the IP address for
the name requested. It may have to do this multiple times.
• DNS can say, "I don't know the IP address for the domain you
requested, but here's the IP address for a name server that knows more
than I do."
• DNS can return an error message because the requested domain name is
invalid or does not exist.
DNS- Applet
Applet
Game Time - Some Tools
• nslookup
• dig
• host
HTTP- Hypertext Transfer Protocol
• HTTP is an application-level protocol
• HTTP is used to transfer data on the
Web.
• HTTP led to the establishment of
the World Wide Web in 1990 by English
physicist Tim Berners-Lee.
How does HTTP work?
Request methods
• GET
• Requests a file from the server.
• POST
• Submits data to be processed (e.g., from
an HTML form) to the identified resource.
• HEAD
• Same as GET without the body.
Request methods (Cont.)
•
•
•
•
•
TRACE
DELETE
PUT
OPTIONS
CONNECT
Tools
• wget
HTML - Hyper Text Markup Language
How can you help the Editor edit the newspaper?
HTML
- Practical Example:
- Start an editor.
- Simple HTML (Header, Body).
- Start adding components (Image,
Headings, Form)
- Design a form response page.
XHTML/HTML
• XHTML is a stricter and cleaner version of HTML:
• XHTML Elements Must Be Properly Nested.
<b><i>This text is bold and italic</i></b>
• XHTML Elements Must Always Be Closed.
<p>This is a paragraph</p>
<p>This is another paragraph</p>
• Empty Elements Must Also Be Closed
A break: <br />
• XHTML Elements Must Be In Lower Case:
<body><p>This is a paragraph</p></body>
• XHTML Documents Must Have One Root Element
All XHTML elements must be nested within the
<html> root element.
The List Goes On...
CSS
• HTML was never intended to contain
tags for formatting a document.
• Imagin you have to edit every HTML
page every time you want to change the
style. NIGHTMARE!
• Do not worry CSS to the rescue.
CSS
The CSS syntax:
selector {property:value}
Example:
p {font-family:"sans serif"}
You can also group selectors:
h1,h2,h3,h4,h5,h6 { color:green}
You can select by Class:
h1.center{text-align:center}
<h1 class="center">
This heading will be center-aligned
</h1>
Comments: /*This is a comment*/
Example
- Practical Example:
-
Start an editor.
Create a css file.
Add simple modifications to the style.
Modify the HTML file.
Style the HTML form from previous
example.
- Display the file on the browser.
Resources
• HTML, XHTML and CSS tutorials:
• http://www.w3schools.com
• Web Standards
• http://www.w3.org/
• Validations
• http://validator.w3.org/
• Free CSS Templates:
• http://www.csszengarden.com/
Resources
• List of all the css properties:
• http://htmlhelp.com/reference/css/all-properties.html
• Domain Name Service DNS
• Computer Networks, Fourth Edition By
Andrew S. Tanenbaum Chapter 7
• HTTP Protocol RFCs
• http://www.w3.org/Protocols/
• Free CSS Templates:
• http://www.csszengarden.com/
Exercise
• Using
nslookup,
host
and
dig
Commands, find the IP addresses for the following
URLs:
• www.bham.ac.uk
• www.google.co.uk
• www.cs.bham.ac.uk
• www.w3school.com
• The first 5 students to send me the correct
commands and their output will get a prize
each.
Conclusion
• There are many types of technologies which
are used to support the world wide web and
more are being developed all the time.
• There are many resources to learn
more about each of these Technologies.
• Designing web pages is fun.