tag.

– We use attributes to give a checkbox a name and to indicate if it is ticked by default .

•••

twentythree.html

Radio Buttons

• A group of radio buttons is specified by using a separate

tag for each button.

– To indicate that the buttons belong to the same group, they are all given the same name .

– For each button we use the attribute

VALUE

to give the value of the button.

Later we will see how this value is used.

– We give the text to be displayed beside the button after the tag.

– We can also use the attribute

CHECKED

to specify which of the buttons is checked.

•••

twentythree.html

Buttons

• We have two types of regular buttons submit and reset buttons.

– These buttons can be specified by using the

tag.

– By default, these buttons are displayed with the text "

Submit Query

" and "

Reset

". – We can use the attribute

VALUE

to change the value displayed.

– We do not need to give buttons names.

•••

twentythree.html

Hidden Input

• It is sometime useful to have controls that are hidden .

– i.e., controls that have a name and a fixed value that cannot be changed by the user.

– In fact, such controls are hidden and do not appear on the display.

– These controls can be specified by using the

tag.

– E.g. a posting ID

•••

Actions

•

Q: A form allows us to fill out text fields, check buttons etc., but what can we do with the data that a user enters?

• A: We can submit it to a server .

– i.e., we send the data entered into the form to a server , this interprets the data and sends back results.

– This is achieved by pressing any submit button .

– If we press any reset button , then the controls on the form are reset to their default values.

• For this process to work, we must specify: – The address of a server.

• For this we use a Uniform Resource Locator ( URL ).

• We will discuss the structure of URLs later in the course.

– How the data should be sent to the server.

• There are two methods POST and GET .

–

With GET, the data is appended to the end of the URL and sent to the server.

–

With POST, the data is sent separately from the URL.

• In both cases, the browser sends pairs current value .

consisting of the name of the control and its

FTP

File Transfer Protocol (FTP)

One of the most popular uses of the internet is to download/ upload files that is transfer files from a computer on the internet to your computer and vise versa.

– Various types of files (executable programs, graphics, audio, text) – Different types of FTP Sites all use Logon & Password • anonymous FTP sites (anonymous & email address as passwords) or automatic logon • Private FTP Sites (account number & password) – FTP runs on a Client / Server model.

• windows has a client command-line FTP program, but other software programs such as WS_FTP95 are simpler and easier to use.

• FTP Daemon runs on FTP Server handling all FTP transactions, the deamon will ask for an account number (or username) and password • Connection command link is opened between your computer and the FTP server when you logon to a FTP site and remains open until explictly closed by user • Downloading opens a second link –

data connection link

modes ASCII mode or Binary mode) (two – Data Connection link closed auotmatically after data has been downloaded • File compression used for large file

Telnet

TELNET

What is Telnet

The Telnet protocol is often thought of as simply providing a facility for remote logins to computer via the Internet. Telnet follows a Client / Server Model. You run a piece of software on your own PC (the client) to use the

resources

of a

distant

Server Computer (host computer).

T

elnet enables the user of a PC to login to a host computer at another site on the Internet. The user's PC then acts as a dumb terminal attached to the remote host. To use telnet you need to know the internet address of the host whose resources you want to use.

Some practical uses of Telnet include:

– Telnet is a Protocol that lets you use the power of the Internet to connect you to databases, library catalogs, and other information resources around the world. – Telnet lets you connect directly to another computer on the Internet and run programs on the computer (provided you have access permission (Username and Password). – Theoretically you have a lot of power at your fingertips. Your dumb terminal or very old PC can run programs on the worlds most powerful computers.

Telnet may have a few practical drawbacks also

– Telnet is old. It is text based only. – You have to know how to run programs at the other end. – Different Computer Systems – Different Commands, Directory Structures – Different Language -- Programming and ``spoken''

How the Internet Works

A Basic Guide to the Internet

What is the Internet ?

• Network of Networks • Computers communicate over the internet using TCP/IP (Transmission Control Protocol / Internet Protocol) • Client / Server Architecture

Funding the Internet

• Funded through Agencies ( National Science Foundation, Government agencies pay for some high speed backbones)

Who Runs the Internet –

establish standards overseen by a variety of groups to • • • • Internet Society (A private non-profit organisation) IETF (Internet Engineering Task Force) TCP/IP W3C (World Wide Web Consortium) MIT Private Companies oversee the registering of Internet Domains.

Underlying Architecture of the Internet

We are going to look at the following Architectures – How computers send data across the internet – How TCP/IP works – How Internet Addresses and Domain Names Work – How Routers Work

How Computers send data across the Internet

– TCP/IP • Hubs • Bridges • Gateways • Repeaters • Routers

How Computers Send Data

• TCP – breaks the data into packets • Computer sends those packets – local Network, Internet Service Provider (ISP), or On-Line Service • Packets then travel through many levels of networks, computers, and communication lines before reaching their final destination.

• Variety of hardware transmits the data between various networks. Five most important pieces of hardware are: –

Hubs

used to link groups of computers to one another and let computers communicate with each other.

– –

Bridges

Links one LAN with another LAN

Gateways

like Bridges but allow communication between different types of networks –

Repeaters -

when data travels over long distances, the signal sending the data can weaken over long distances, repeaters are used to amplify the data at intervals –

Routers

different networks. It examines packets for destination and selects the quickest route taking into account traffic on the internet.

come into play when data is being sent between two

How TCP/IP Works

• The Internet is a packet switched network – Messages broken into a number of packets + data to help the packet find its way through the Internet (TCP) – Series of switchs called routers (IP) ensure that the packages arrive at their correct destination – Packets are sent over many different routes at the same time • Hardware restriction – data broken into packets of 1,500 bytes each.

• Packet given header with order of packet & checksum (based on the amount of data in packet) • Each packet is put into separate IP envelopes, which contain addressing information telling the Internet where to send the packet • IP envelopes contain headers that contain information: – Senders Address – Destination Address – Amount of time packet should be kept

– As packets sent across Internet, routers examine IP envelope destination addresses and determine the most efficient route for sending each packet. (Packets can arrive out of order) – Packets arrive at their destination and TCP calculates the checksum for each packet (error checking) – When all noncorrupt packets arrive at their destination TCP assembles them into their original form • Winsock software (TCP/IP stack or Socket) serves as an intermediary between the Internet and PC • Two ways to connect to the Internet and use the TCP/IP protocol – Direct connection (via LAN, cable modem, Digital Subscriber Line (DSL) line – needs a network card & hardware driver) – Dialing connection (Modem) • Serial Line Internet Protocol (SLIP) • Point-to-Point Protocol (PPP)

Sample Internet Connections

The basic idea of communication via the internet is to have two remote sites or computers connected together via a network or transmission line

Internet Addresses and Domain Names

The heart of how the Internet works is the Domain Name System (DNS), the way in which computers can contact each other and do things such as e-mail or display web pages. If a user wants to contact a specific location – e.g. visit Web pages they type in the address (URL) –

www.computing.dcu.ie

While numbers are convenient for machines, they are not for human beings, therefore humans use names for addresses and TCP/IP uses the DNS (Domain Name System) application (Name Servers) to provide name-to-address translation. The DNS translates the plain english address into a series of numbers called an IP address, 136.2.6.11.XXX

An IP address marks the location on the internet similar to a house number and street address. All computers connected to the Internet must have an IP address.

Two types of IP address exist

– Static IP Addresses – Dynamic IP Addresses

Dynamic IP Addresses

DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPPACK

DHCP Server

Domains can be organised in hierarchy

–

Major Domains and minor domains

• www.computing.dcu.ie

–

.ie

is the major domain – –

.dcu

is the Dublin City University Domain

.computing

is the computer applications server –

Some Common Major Domain Names in the United States are:

.edu

for educational institution, usually a university .com

for a commercial business .gov

.mil

for a government department or agency for a military unit .org

for a non-profit organisation Following introduced to reduce the load on .com domain. Not all are in use yet but they officially registered.

.areo

for aviation industry .info

for general use .biz

for business .museum for museums .pro

for professionals .coop

.name

for cooperatives for individuals

As networks in other countries were connected to the Internet, they were assigned their own domain names – – – ie ca au Ireland Canada Australia – – uk de United Kingdom Germany In total there are more than 250 top-level domain names The IP address is a 32 bit number but is now being expanded to a 128 bit number to allow for more IP addresses to be accommodated.

Since networks vary in size, there are four different address formats or classes to consider when applying for a network number:

Class A

addresses are for large networks with many devices.

Class B

addresses are for medium-sized networks.

Class C

addresses are for small networks (fewer than 256 devices).

Routers

How Routers Work

Routers are traffic cops of the Internet. They ensure that data gets to its final destination via the most efficient route.

•Routers check the IP envelopes for the destination address •Calculate the best route and then send the package on its way – Router sends packet to another router closer to its final destination. This router in turns forwards the packet on to a router closer to the final destination (hops).

– Each router has to consider factors such as • Traffic congestion • Number of hops (IP packets carry a segment that holds the max hop count)

•

Routers have two or more physical ports

– Input port (routing process run, routing table) – Output port (results from the routing process defines with output port the routes the packets should follow) – Input queue (holding area for input port, queue capacity exceeded – lost data)

Routing Table

– Static – simpler and specifies specific paths for packets – Dynamic – packets can have multiple routes to their final destination (table changes as network conditions change).

Two broad type of routing protocols exist

– Interior (Gateway Routing Information Protocol (IGRP)) – Exterior (Exterior Gateway Protocol (EGP))

Logical and Physical Styles

Logical Styles

(indicates the way text is used)

Emphasis ... Basic emphasis,

typically displayed in italics

. Strong ... Provides strong emphasis,

usually bold

. Citation ... Specifies a citation such as book titles, references, etc.

Usually displayed in italics

. Code ... Used when displaying program code. I'm using it here whenever I give examples of the tags. Sample Output ... Used when displaying sample output from programs.

Keyboard Input ... Use for text input by the user. Variables ... Used for

variables or arguments to commands

. WWW Indicates an

abbreviated form

(e.g., WWW, HTTP, URL, etc.). Definition ... Used for

definitions

. Address

the author's address and other contact details, and often force a line break before and after.

Block Quote

...

Physical Styles

Bold ... Used to

bold type

(generally the same as Strong). Italic ... Used to

italic type

(generally the same as Italic). Underline ... Underlines text (some old browsers don't do this) Typewriter Text ... Displays a mono-spaced names or HTML code. font, usually used for variable Strike-Through ... Strikes through text as if you're crossing it out. Big ... Displays text in a big font.

Small ... Displays text in a small font. Subscript _... Places text in subscript style . Superscript ^... Places text in superscript style . Preformatted

 ... 

Center

Blink ... off.

An annoying tag that makes the text blink on and

Incremental Graphics and Thumbnail Images

Make slow image downloads less annoying

To make slow downloads less annoying you can use – –

Incremental Graphics Thumbnail Previews Incremental graphics: Interlaced GIFs

. The interlaced GIF file produces its display in four passes.

–

First pass

produces produces an image based on every 10 line of pixels th – –

Second pass

shows every fifth line

Third pass

everyline is filled in –

Fourth pass

the complete image is displayed JPEG uses a

Progressive JPEG

file format – works similar to Interlaced GIFs

Thumbnail Previews

A thumbnail preview (thumbnail Sketch) is a clickable bandwidth efficient version of a larger graphics image which contains a link to the original image. Click on this picture to view the original image (79.6KB ) The important thing to note when creating a thumbnail image is that you are creating a second image in a smaller size rather than just adjusting the width and height parameteres in the HTML document.

1.

How to Create Thumbnail Image using Paint Shop Pro (PSP)

Open the Image that the thumbnail is to linked to, in PSP.

2.

From the image menu select the RESAMPLE option, if the resample option is greyed out, then and only then you should select the resize option.

3.

Select the width that you want the thumbnail to be and make sure that the maintain aspect ration box is selected.

4.

Click the OK button to resize the image.

5.

In the file menu select the save as option and save the file with a different name from the original file.

Frames

twenty.html

Components of the Internet

Components of the Internet

The Internet is comprised of many components such a Email, FTP and Usenet News. The World Wide Web is simply one of these components.

•

World Wide Web

(World Wide Web is like an Internet Library with millions of books) • FTP • E-mail & E-mail Discussion Groups • Telnet • Usenet News • HTTP • Chat & Instant Messenging

What is the World Wide Web?

One simple definition of the WWW is:

The WWW is a Hypertext Information System Hypertext browsing: •Non-Linear structure (not a book) •You read what you want next •Click on Hypertext links to navigate the WWW

Features of the WWW are:

•Graphical •Easy to use •Cross Platform •Distributed •Dynamic •Interactive (forms, Java)

What is Hypertext?

Hypertext is a method of presenting information, where selected words in text can be expanded at any time to provide other information about the word. Those words are

links

to other documents which may be text, file, pictures etc., Links can be identified by highlighted (underlined) and coloured text.

HTML

(Hypertext Markup Language) is the language used to create WWW pages and associated links.

URLs

(Uniform Resource Locators) are the addresses of the WWW pages

http://www.computing.dcu.ie/~cgreene/index.html

To view / read the WWW pages you must have a special application i.e. a

web browser

A Web browser is an application program that provides a way to look at and interact with all the information on the World Wide Web. Technically, a Web browser is a client program that uses the Hypertext Transfer Protocol (HTTP) to make requests of Web servers throughout the Internet on behalf of the browser user and then displays web pages by interpreting the HTML that is used to build thses pages on the web.

To

publish

and

deliver information

on the web you need a

web Server

A web server is a program that runs on web sites and is responsible for replying to a web browsers request for files

The Web works on a client / server model in which client software (Browser) runs on a local computer. The Server software runs on a web host. To use the web you must make an Internet connection and then launch your web browser.

– – Type the URL you want to visit Browser makes a connection to a server using HTTP protocol – The Server then • • • Accepts the connection Sends the file contents back to browser, and Closes the connection – Web browsers interpret the html pages and display them on screen, they can also display programs, animation, graphics, video-conferencing, download files etc., Browsers also allow you to create and publish web pages.

• • Entire suites built around browsers including modules for reading newgroups, for sending/receiving e-mail Multimedia files – helper applications (plug-ins)

Broswers and Search Engines

Web Browsers

Common Web Browsers

– Mosaic – first graphical browser – Netscape Navigator (First commercial browser, originally known as Mosaic Navigator as originated from Mosaic) – Internet Explorer (Microsoft’s browser) – Lynx – text only browser

Common Errors with Browsers

– Server Does Not Have A DNS Entry – 503 Service Unavailable – 403.9 Access Forbidden – Too Many Users are Connected – Spinning Hourglass – 404 Not Found – 401 Unauthorised and 402 Forbidden

•

Search Engine

– Lycos & Alta Vista (index based engines) – Yahoo (directory based search engine) – Google (index based) – Ask Jeeves (natural language search engine)

Index based search Engines

– built by computer robot programs (“Spiders") -- not by human selection – NOT organized by subject categories -- all pages are ranked by a computer algorithm – contain full-text (every word) of the web pages they link to -- you find pages by matching words in the pages you want – huge and often retrieve a lot of information -- for complex searches use ones that allow you to search within results (sub searching)

•

Meta Search Engines

• Metacrawler • copernic Utilities that search more than one search engine and/or subject directory at once and then compile the results in a consolidating uniform format and listing. Some offer added value features like the ability to refine searches, customize which search engines or directories are queried.