Download Notes1 - DCU School of Computing

How the Internet Works
The World Wide Web is the most
popular part of the Internet by far.
The Web allows rich and diverse
communication by displaying text,
graphics, animation, photos, sound
and video. The Web physically
consists of your personal computer,
web browser software, a
connection to an Internet Service
Provider, computers called servers
that host digital data, and routers
and switches to direct the flow of
information.
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 address of the WWW pages
http://www.computing.dcu.ie/~jkernan/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)
Web Browsers
Common Web Browser
– 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 Error 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.
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 (two
modes ASCII mode or Binary mode)
– Data Connection link closed auotmatically after data has
been downloaded
• File compression used for Large file
Electronic Mail (E-mail)
• Email uses the TCP/IP protocol
• SMTP (Simple Mail Transfer Protocol)
– File attachments (binary Files)
• MIME (Multipurpose Internet Mail Extension)
• Uuencode
• How e-mail is delivered
– TCP/IP
– Gateways translate email formats from one network to the
next
– Routers internal / external (firewall)
• E-mail Software
– Mail Clients – Eudora, MS Exchange
• Create folders, search through messages, address books,
mailing lists
– Most e-mail software reads HTML- based pages (links)
• Mail Server
• Encryption for E-Mail Privacy
– Public & Private Keys most common form of encryption
• How E-Mail Spam Works
– Spam is Unsolicitied Junk E-mail
– Blocking Spam
• Write back to Spammer and asked to be removed
• E-mail Software filters e-mail addresses
• ISP like AOL block spammers from sending bulk mail to
subscribers
• Ensure that your email address isn’t added to spam list
– Notify e-mail directories to remove your e-mail
address
– When posting to usenet newsgroups edit your header
so that it doesn’t contain your e-mail address
Email: Advantages and Disadvantages
Advantages
• Convenience
• Speed
• Inexpensive
• Printable
• Reliable
• Global
• Generality
Disadvantages
• Misdirection
• Interception
• Forgery
• Overload
• Junk
• No response
Email Addresses
Basic form of an email address is
[email protected]
E-Mail Discussion Groups
Mailing Lists are a way for groups of people to have public
Discussions via e-mail.
Mailing lists can be moderated or unmoderated.
– Moderated (screened by the list moderator)
– Unmoderated (wide open list)
Users must subscribe to a mailing list to join, this is achieved by
sending a subscribe message to the computer known as a list server
(Listserv). It reads your e-mail and automatically subscribes to the
list. You can unsubscribe from the list in the same way.
The Mailing List is a database of e-mail addresses who have
subscribed.
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).
Telnet 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''
Newsgroups
USENET is the worlds biggest electronic discussion forum, provides
a way for messages to be sent among computers across the entire
Internet. People from all over the world participate in discussions
on thousands of specific areas of interest called newsgroups
– Global bulletin Board and discussion area.
– Usenet and all messages are stored on a Usenet Server
– Different Hierarchies of Newsgroups exist.
• Subcategories
– Internet site Administrator decides which newsgroup to carry
– Special software to particiapate in newsgroups.
– Moderated & Unmoderated Newsgroups
• Moderated – human moderator
• Unmoderated – automatically posted to everyone
– Subscribe \ Unsubscribe \ Read
– Pictures and multimedia files can be posted – files must be
specially encoded and then downloaded and unencode to
view them.
HTTP (Hypertext Transfer Protocol)
HTTP is a set of rules, or protocols which governs the transfer of
hypertext between two or more computers.
The World Wide Web encompasses the universe of information that
is available via HTTP.
– HTTP is based on the client/server principle.
– HTTP allows "computer A" (the client) to establish a
connection to "computer B" (the server) and make a request.
The server accepts the connection initiated by the client and
sends back a response.
– An HTTP request identifies the resource that the client is
interested in and tells the server what "action" to take on the
resource.
When a user selects a hypertext link, the client program on their
computer uses HTTP to contact the server, identify a resource, and
ask the server to respond with an action. The server accepts the
request, and then uses HTTP to respond to or perform the action.
For example, when you select a hypertext link, you are identifying a
particular resource, and asking the server to send it back to your
computer in a format that your computer can display.
Chat & Instant Messenging
There are three commonly used types of chat. They are Instant
Messaging, ICQ, and IRC.
1. Instant Messaging
Instant messaging (IM) is one of the most popular forms of chat.
Most of the time, instant messaging is just between two people,
although most IM software can handle group chats (with 3 or more
people.) User must choose a nickname.
The most popular form of IM is AOL Instant Messenger. AOL IM
allows anyone who downloads their free software to choose a
nick(name) (AOL calls their nicks Screen names) and 'chat' with
anyone else with AOL IM or the America Online Service itself.
Nearly everyone with America Online is reachable by AOL IM. If
you see an AOL e-mail address, you can often cut off the @aol.com
to get the IM screen name to contact the person directly.
•No two people with AOL IM or AOL can have the same screen name.
•AOL IM is free to download http://www.aol.com/aim/home.html. The
software and web site have detailed instructions on how to get
started.
2. ICQ ("I seek you")
ICQ is an extremely useful communication program. It allows you to
send messages (chat), send files, send URLs, and more to anyone
else with ICQ.
– When you download ICQ (which is free) you are assigned an
ICQ number. Two or more people using ICQ can have the
same nickname, but no two people can have the same ICQ
number.
3.
•
•
•
•
IRC - Internet Relay Chat
Unlike AOL IM and ICQ, IRC (Internet Relay Chat) is not owned
by any company. To use IRC, you need an IRC client program.
IRC has many networks that are completely separate from one
another. Some popular networks include The Undernet, EFnet,
and DALnet. If you want to chat with someone on IRC, you need
to make sure that both of you are on the same network (you can
set it and change it each time your start your IRC program.)
IRC has very few rules. You are allowed to say pretty much
anything to want to pretty much anyone you want. For example,
if you are on AOL, and someone keeps bothering you, you could
report that screen name to the AOL company. There is no high
authority on IRC, but there is still an ignore option to use if
someone annoys you.
Once you connect to an IRC network with an IRC client program,
there are often thousands of rooms (they are called channels) on
nearly any topic, and tens of thousands of different people to
chat with.