DNS – HTTP – DHTML - CSS ICW Lecture 5 Hasan Qunoo

DNS – HTTP – DHTML - CSS

ICW Lecture 5Hasan Qunoo

DNS - Domain Name System

School of Computer ScienceThe University of BirminghamEdgbastonBirminghamB15 2TTUnited Kingdom

.uk

.ac

.bham

.cswww


• 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.


• 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?)

http://www.bham.ac.uk/

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

http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/dns/dns.html

Game Time - Some Tools

• nslookup

• dig

• host

http://docsrv.sco.com/NET_tcpip/dnsC.nslook.html



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.

This text is bold and italic

• XHTML Elements Must Always Be Closed.

This is a paragraphThis is another paragraph

• Empty Elements Must Also Be ClosedA break: 

• XHTML Elements Must Be In Lower Case:<body>This is a paragraph</body>

• XHTML Documents Must Have One Root ElementAll 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.

CSSThe 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/

http://www.w3schools.com/

http://www.w3.org/

http://validator.w3.org/

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.