Web Programming Step by Step

Lecture 1
Internet/WWW; HTML Basics

Reading: Chapter 1; 2.1

Except where otherwise noted, the contents of this presentation are Copyright 2010 Marty Stepp and Jessica Miller.

The Internet

• Wikipedia: http://en.wikipedia.org/wiki/Internet
• a connection of computer networks using the Internet Protocol (IP)
• What's the difference between the Internet and the World Wide Web (WWW)?

Brief history (1.1.1)

• began as a US Department of Defense network called ARPANET (1960s-70s)
• initial services: electronic mail, file transfer
• opened to commercial interests in late 80s
• WWW created in 1989-91 by Tim Berners-Lee
• popular web browsers released: Netscape 1994, IE 1995
• Amazon.com opens in 1995; Google January 1996
• Hamster Dance web page created in 1999

Key aspects of the internet

• subnetworks can stand on their own
• computers can dynamically join and leave the network
• built on open standards; anyone can create a new internet device
• lack of centralized control (mostly)
• everyone can use it with simple, commonly available software

People and organizations (1.1.2)

• Internet Engineering Task Force (IETF): internet protocol standards
• Internet Corporation for Assigned Names and Numbers (ICANN):
  decides top-level domain names
• World Wide Web Consortium (W3C): web standards

Layered architecture (1.1.3)

The internet uses a layered hardware/software architecture (also called the "OSI model"):

• physical layer : devices such as ethernet, coaxial cables, fiber-optic lines, modems
• data link layer : basic hardware protocols (ethernet, wifi, DSL PPP)
• network / internet layer : basic software protocol (IP)
• transport layer : adds reliability to network layer (TCP, UDP)
• application layer : implements specific communication for each kind of program (HTTP, POP3/IMAP, SSH, FTP)

Internet Protocol (IP)

• a simple protocol for attempting to send data between two computers
• each device has a 32-bit IP address written as four 8-bit numbers (0-255)
  
• find out your internet IP address: whatismyip.com
• find out your local IP address:
  • in a terminal, type: ipconfig (Windows) or ifconfig (Mac/Linux)

Transmission Control Protocol (TCP)

• adds multiplexing, guaranteed message delivery on top of IP
• multiplexing: multiple programs using the same IP address
  • port: a number given to each program or service
  • port 80: web browser (port 443 for secure browsing)
  • port 25: email
  • port 22: ssh
  • port 5190: AOL Instant Messenger
  • more common ports
• some programs (games, streaming media programs) use simpler UDP protocol instead of TCP

Web servers and browsers (1.2.1)

• web server: software that listens for web page requests
  • Apache
  • Microsoft Internet Information Server (IIS) (part of Windows)
• web browser: fetches/displays documents from web servers
  • Mozilla Firefox
  • Microsoft Internet Explorer (IE)
  • Apple Safari
  • Google Chrome
  • Opera

Domain Name System (DNS) (1.2.2)

• a set of servers that map written names to IP addresses
  • Example: www.cs.washington.edu → 128.208.3.88
• many systems maintain a local cache called a hosts file
  • Windows: C:\Windows\system32\drivers\etc\hosts
  • Mac: /private/etc/hosts
  • Linux: /etc/hosts

Uniform Resource Locator (URL)

• an identifier for the location of a document on a web site
• a basic URL:

  http://www.aw-bc.com/info/regesstepp/index.html
  ~~~~   ~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~
  protocol    host                path
  

• upon entering this URL into the browser, it would:
  • ask the DNS server for the IP address of www.aw-bc.com
  • connect to that IP address at port 80
  • ask the server to GET /info/regesstepp/index.html
  • display the resulting page on the screen

More advanced URLs

• anchor: jumps to a given section of a web page

  http://www.textpad.com/download/index.html#downloads
  

  • fetches index.html then jumps down to part of the page labeled downloads
• port: for web servers on ports other than the default 80

  http://www.cs.washington.edu:8080/secret/money.txt
  

• query string: a set of parameters passed to a web program

  http://www.google.com/search?q=miserable+failure&start=10
  

  • parameter q is set to "miserable+failure"
  • parameter start is set to 10

Hypertext Transport Protocol (HTTP) (1.2.3)

• the set of commands understood by a web server and sent from a browser
• some HTTP commands (your browser sends these internally):
  • GET  filename : download
  • POST filename : send a web form response
  • PUT  filename : upload
• simulating a browser with a terminal window:

  $ telnet www.cs.washington.edu 80
  Trying 128.208.3.88...
  Connected to 128.208.3.88 (128.208.3.88).
  Escape character is '^]'.
  GET /index.html
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 ...">
  <html>
  ...
  

HTTP error codes

• when something goes wrong, the web server returns a special "error code" number to the browser, possibly followed by an HTML document
• common error codes:

  Number	Meaning
  200	OK
  301-303	page has moved (permanently or temporarily)
  403	you are forbidden to access this page
  404	page not found
  500	internal server error
  complete list

Internet media ("MIME") types

• sometimes when including resources in a page (style sheet, icon, multimedia object), we specify their type of data

  MIME type	file extension
  text/html	.html
  text/plain	.txt
  image/gif	.gif
  image/jpeg	.jpg
  video/quicktime	.mov
  application/octet-stream	.exe

• Lists of MIME types: by type, by extension

Web languages / technologies (1.2.4)

• Hypertext Markup Language (HTML): used for writing web pages
• Cascading Style Sheets (CSS): stylistic info for web pages
• PHP Hypertext Processor (PHP): dynamically create pages on a web server
• JavaScript: interactive and programmable web pages
• Asynchronous JavaScript and XML (Ajax): accessing data for web applications
• eXtensible Markup Language (XML): metalanguage for organizing data
• Structured Query Language (SQL): interaction with databases

Hypertext Markup Language (HTML) (2.1.1)

• describes the content and structure of information on a web page
  • not the same as the presentation (appearance on screen)
• surrounds text content with opening and closing tags
• each tag's name is called an element
  • syntax: <element> content </element>
  • example: <p>This is a paragraph</p>
• most whitespace is insignificant in HTML (ignored or collapsed to a single space)
• we will use a stricter, more standard version called XHTML

Structure of an HTML page (2.1.2)

<html>
	<head>
		information about the page
	</head>

	<body>
		page contents
	</body>
</html>

• the header describes the page and the body contains the page's contents
• an HTML page is saved into a file ending with extension .html
• but this is not quite the syntax we'll use... (see next slide)

Structure of an XHTML page (2.1.2)

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
	<head>
		information about the page
	</head>

	<body>
		page contents
	</body>
</html>

• the DOCTYPE and xmlns additions tell the browser to interpret our page's code as XHTML, the latest/greatest standard version of the language

Page title: <title>

describes the title of the web page

<title>Chapter 2: HTML Basics</title>

• placed within the head of the page
• displayed in the web browser's title bar and when bookmarking the page

Paragraph: <p> (2.1.3)

paragraphs of text (block)

<p>You're not your job.
You're not how much money you have in the bank.
You're not the car you drive.   You're not the contents
of your wallet. You're not your         khakis.  You're
   the all-singing, all-dancing crap of the world.</p>

• placed within the body of the page
• more paragraph examples

Headings: <h1>, <h2>, ..., <h6>

headings to separate major areas of the page (block)

<h1>University of Whoville</h1>
<h2>Department of Computer Science</h2>
<h3>Sponsored by Micro$oft</h3>

• More heading examples

Horizontal rule: <hr>

a horizontal line to visually separate sections of a page (block)

<p>First paragraph</p>
<hr />
<p>Second paragraph</p>

• should be immediately closed with />