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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.