how does the internet work l.
Skip this Video
Loading SlideShow in 5 Seconds..
How does the Internet work ? PowerPoint Presentation
Download Presentation
How does the Internet work ?

Loading in 2 Seconds...

play fullscreen
1 / 28

How does the Internet work ? - PowerPoint PPT Presentation

  • Uploaded on

How does the Internet work ? What we will cover What is the Internet Client/server examples Deciphering alphabet soup: ISP, DHCP, DNS, http, https, HTML, smtp, pop3, telnet, ftp, sftp, ssh Network security: How to send anonymous emails How to hack into a system How a Firewall works

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'How does the Internet work ?' - johana

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
what we will cover
What we will cover
  • What is the Internet
  • Client/server examples
  • Deciphering alphabet soup: ISP, DHCP, DNS, http, https, HTML, smtp, pop3, telnet, ftp, sftp, ssh
  • Network security:
    • How to send anonymous emails
    • How to hack into a system
    • How a Firewall works
    • Encrypted communications
internet history
Internet History

The Internet is a global network of interconnected computers, enabling users to share information along multiple channels.

  • Started in 1969 as ARPANET funded by DARPA (Defense Advanced Research Projects Agency)
  • Build as packet switching network to recover from a nuclear attack by automatically rerouting data through surviving links

“When I took office, only high energy physicistshad ever heard of what is called the World WideWeb... Now even my cat has it's own page.”Bill Clinton

the user perspective
The User Perspective
  • have little laptop at home in NJ
  • want to exchange information with big server in California (or Japan or …)
basic setup
Basic Setup
  • Each computer must have a unique identifier
    • IP number and IP name
  • Computers must be able to exchange data (electrons, photons, drum beats)
    • Wireless cards, fiber optics, or Ethernet connections
    • Unit of data is “bit” (“zero” or “one”, on/off, 2 states)
  • Everyone involved must speak the same language
    • TCPIP (Transmission Control Protocol/Internet Protocol)
network member identifier
Network Member Identifier
  • Every computer on the Internet has at least one unique identifier, usually two:
    • IP Number: #.#.#.#, where # is an 8 bit number
      • What is the range for each sub-number?
      • How many machines can be on the Internet?
      • What is your computer’s IP number?
    • IP Name for easy reference
      • What is your computer’s IP name?
    • Homework:
      • What is the IP number of “google”
      • What is the IP number of “”
      • What is the IP name of “”

Router A

Router B



fiber optic lines

ISP: Internet ServiceProvider

DHCP: Dynamic HostConfig. Protocol

DNS: Domain Name Server


the data ip packet
The Data: IP Packet


the data ip packet10
The Data: IP Packet


the protocol
The Protocol

A mutually agreed-upon convention or standard that controls or enables the connection, communication, and data transfer between computing endpoints.

  • Regulates the data exchange and interpretation
  • Defines who says what at which time
  • Defines how to interpret data that is exchanged
  • Regulates what constitutes an error and what to do if one occurs
client server model
Client – Server Model
  • Communication on the Internet usually takes place between a client and a server program/computer:
    • Server program: program without a user interface running on a “large” computer with access to many resources (also called the server computer)
    • Client program: a program with extensive interface capabilities but few resources running on a “small” computer (also called the client computer)
  • One server computer usually run several server programs, each of which can service multiple client programs simultaneously
client server example 1
Client – Server Example (1)

I want to view the main web page from

  • Server computer:
  • Client computer:
  • Server program: web server program (httpd) running on with access to lots of stored web pages
  • Client program: Internet Explorer or Firefox with extensive formatting and display capabilities but no data to display
client server example 2
Client – Server Example (2)

I want to view the main web page from

  • Client: start IE and enter:
    • Client sends packet to DNS: who is
    • DNS sends packet back: =
    • Client sends packet to give me main page
  • Server: receives request for page from
    • Retrieves the web page from disk (or database)
    • Sends data to here is the data
  • Client: formats data and display it nicely
client server example 3
Client – Server Example (3)

I want to view the main web page from

  • Client: sends “give me main page”Server: sends data to
    • http (Hypertext Transport Protocol): regulates how a web server and client communicate
  • Client: formats data and display it nicely
    • HTML (Hypertext Markup Language): defines how text is supposed to look and where to place it
client server example 4
Client – Server Example (4)

I want to view the main web page from

client server example 5
Client – Server Example (5)


  • universal text client used to connect to another computer and work on that computer in text-based mode
  • usually connects to a “telnet server” but can also connect to any server computer and any server program
  • shows text data in ‘raw’ unformatted form
client server telnet
Client – Server: Telnet

A Telnet client is build into Windows:

  • Click on “Start”
  • Pick “Run …” and type “cmd”
  • Type “telnet”

if you get error message in Vista, open Control Panel, select “Programs”, click “Turn Windows features on or off”, and check “Telnet client” (not “Telnet server”). Then try again.

  • To open a connection to, type:

open (will this work – why not?)

  • Optional: to save a log of your session, type:

set logfile log.txt

telnet ing to a web server
Telnet’ing to a Web Server
  • Start “telnet” and type:

open 80

  • Next type carefully and without errors (you might not see what you type on the screen – type anyway, including the empty line and capitals):

GET / HTTP/1.0

  • You have issued a request according to the Hypertext Transfer Protocol, version 1.0, for the root web page /
how email works
How email works

Email systems have two parts, and consequently work with two server programs and two protocols:

  • Retrieving email

uses either pop3 (Post Office Protocol version 3 on port 110) or IMAP (Internet message access protocol on port 143)

  • Sending email

uses smtp (simple mail transport protocol on port 25)

  • Capture a web page from a web server
  • Send me an (anonymous) email using Telnet (note that the SHUsmtp server will only allow a connection if you are located on campus – how does it know?)
hacking 101
Hacking 101
  • Identify a target system
  • Find an open port
  • Check which server program services that port
  • Learn about vulnerabilities of that server program
  • Exploit vulnerability for evil purposes
firewall protection
Firewall Protection


  • Every packet transported over the Internet contains the protocol (port), sender address, and destination address
  • A firewall is a device that inspectsevery incoming (and outgoing) packet and includes rules to block data depending on the port, sender, or destination
  • A firewall is typically integrated into a home wireless router
  • Most firewalls do not check the content of a packet
spam protection
SPAM Protection


  • Every Internet packet contains the port, sender, destination, and content (data) – usually unencrypted
  • A SPAM filter is a device that inspectsall email packets and includes rules to block messages depending on the content of the email.
  • A SPAM filter is typically integrated into you ISP or email provider
  • SPAM filters typically read your email!
privacy content protection
Privacy/Content Protection


  • All Internet traffic is public!
  • To protect content, the data portion of IP packets must be encrypted
    • To protect data between your wireless router and your laptop, use e.g. WPA wireless encryption
    • To protect data after it leaves your router, you must use encrypted services (https instead of http or sftp instead of ftp)
    • Note: no standard encryption for email!

Sending email is like passing a postcard along a chain of hundreds of people with a note: “to Jane – do not read if you’re not Jane”


Encryption is usually based on a key that used to encrypt and decrypt a message.

  • Pre-Shared Key (PSK) Encryption:
    • both parties know a single key (e.g. WPA encryption: both your router and your computers know the key)
  • Public Key Encryption:
    • each party has two keys, a public and a private one. They swap public keys: Bob uses Annie’s public key to send her an encrypted message, she can decrypt it using her private key
    • based on difficulty of factoring huge numbers into large primes and ease of multiplying them


public key encryption requires
Public Key Encryption requires:
  • large “semi”-prime number x = p q


  • public key x = 15 => private key is:

Homework: Find factorization of RSA-100, which is1522605027922533360535618378132637429718068114961380688657908494580122963258952897654000350692006139

More Info:

  • Enigma by Robert Harris, and Enigma the Movie