Transcript The internet - Computer and Information Science

Internet and WWW
Lecture 2
Network
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A network is a collection of computers
connected together with special hardware (and
software to manage the network).
The purpose of a network is to share resources,
which can be both physical (hardware) and
digital (software and data).
E.g. Atrium or this room-share printers and
software
Client/Server model
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Client requests a service
Server provides the service
LAN/MAN/WAN
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Local Area Network – communication in
small area such as room or building
 Cables or wireless
Metropolitan Area Network – links 2 or
more LANs within a city
Wide Area Network – communication over
a larger distance
 Fiber optic cable
 Leased phone lines
Protocols
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Protocols are rules that must be followed
so the computers on a network can
communicate.
If different networks use different
technologies and protocols, they are
incompatible. (Like 2 toddlers talking with
neither listening.)
Networks
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Companies and organizations
independently developed networks that
were incompatible.
They (especially the U.S. government) felt a
need for internetworking – communication
between networks.
The Department of Defense funded
research on how to achieve
internetworking.
Robust internetwork
The military had a particular perspective on
internetworking:
 There was a fear of creating a network that
would be dependent on a central "super
computer" or central server.
 That was dangerous, because a central
computer might be wiped out during a
military attack.
Robust internetwork
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So the research was oriented towards
designing a robust internetwork that would
remain functional even if parts of the network
would be - literally - destroyed in war.
The Internet developed as a decentralized
distributed network, with many servers.
ARPANet
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ARPANet was funded by the Dept of
Defense for communications
 it needed to be resistant to attack or
mechanical failure
Eventually it became the Internet
The Internet
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The Internet is a vast, international
network of networks
 the physical connections between
computers vary, but the overall effect
is that computers around the world
can communicate and share
resources
The Internet
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The Internet traces its roots back to 1969,
when the U.S. government sponsored the
first long-distance computer network
Starting with only 4 computers, the
network would eventually evolve into
today's Internet
Robust Internet
The success of this design was evident on
9/11. Phone service was wiped out in lower
Manhattan, but Internet access was available
in the unharmed buildings. Many people
could not contact their family or friends by
phone or cell phone, but were able to send
email messages.
Communications protocol
The major product of the research on
internetworking was the development of a
standard for a communications protocol to
interconnect different networks.
Communications protocol
A communications protocol is a
standardized set of rules regulating
communication between computers. (In
foreign policy, protocol governs rules of
interactions with foreign governments.)
TCP/IP
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The communications protocol used on the
Internet is called TCP/IP. TCP/IP software
enables a computer to communicate with
other computers on the Internet. Any
computer that is connected to the Internet
must have TCP/IP software running on it.
Packet Switching
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In a packet-switching network, messages to
be sent over the network are first broken
into small pieces known as packets
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These packets are sent independently to their
final destination
TCP (Transmission Control Protocol) breaks
data into packets at the source and reassembles
them at the destination
Advantages of Packets
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Sending information in smaller units increases the
efficient use of connections
 Large messages can't monopolize the
connection
 Transmitting packets independently allows the
network to react to failures or network congestion
 routers (computers that direct the flow of
messages) can recognize failures or congestion
and reroute the packet around trouble areas
Advantages of Packets
 Breaking the message into packets can improve
reliability
 Since the packets are transmitted
independently, it is likely that at least part of the
message will arrive (even if some failures occur
within the network)
 Software at the destination can recognize which
packets are missing and request retransmission
IP addressing
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The cornerstone of TCP/IP is IP addressing. Every
computer that is permanently connected to the
Internet is given a unique number, called an IP
address. (Similar idea to the way Social Security
numbers are used to identify people.)
Format of IP address - 146.245.1.4 - 4 groups of
numbers, separated by periods. Each number 0255.
IP addressing
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The total number of IP addresses is 2564.
(0-255 each)
Brooklyn College has IP addresses
beginning with 146.245....
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sci network is 146.245.1.x
its network is 146.245.2.x
There can be at most 256 computers on the sci
network.
IP
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Internet Protocol (IP) relies on each
computer having a unique identifier
(known as an IP address)
Each computer is assigned an IP address by
its Internet Service Provider (ISP)
Datagram
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A datagram is a packet together with its
routing information.
Analogous to a letter inside its addressed
envelepe.
TCP/IP
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Transmission Control Protocol (TCP)
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Internet Protocol (IP)
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controls the method by which messages are broken down
into packets and then reassembled when they reach their
final destination
concerned with labeling the packets for delivery and
controlling the packets’ paths from sender to recipient
Routers are computers that receive packets, access
the routing information, and pass the packets on
toward their destination
DNS
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People don't like to have to remember numbers (e.g.
soc sec #). We like to use names.
The DNS - Domain Name System is used to give
names to computers on the Internet. e.g.
mail.sci.brooklyn.cuny.edu
The parts are separated by periods. There is NO
correspondence between these words and the
numbers in the IP address.
There may be more or less than 4 parts to a DNS
name. IP addresses always have 4 parts.
DNS
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A DNS name can be "read" by going from
the most specific to the more general,
similar to the way a mailing address works.
We might send mail to
John Doe
123 Main Street
Smalltown, ST
DNS
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The DNS name mail.sci.brooklyn.cuny.edu refers
to a computer called "mail" which is part of the
"sci" network, which is part of the Brooklyn
College network, which is part of the CUNY
system, which is an educational institution.
Top-level domains (TLD)
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The top-level domains are categories
 education (.edu)
 government (.gov)
 commercial (.com)
 organizations (.org)
 Military (.mil)
 Network support (.net)
Top-level domains
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There are country codes that are used
as top-level domains, usually 2characters
 .ca for Canada
 .jp for japan
 .ru for Russia
 .il for Israel.
DNS->IP
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Internet software automatically converts DNS
names into IP addresses.
When we specify a location, the software
looks up the IP address and then initiates the
connection.
(Analogy: CEO asks a secretary to get
someone on the phone. The secretary first
looks up the phone number and then makes
the phone connection.)
History of the Web
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The World Wide Web is a multimedia
environment in which documents can be
seamlessly linked over the Internet
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It was proposed by Tim Berners-Lee at the
European Laboratory for Particle Physics (CERN) in
1989
It was designed to facilitate sharing information
among researchers located all over Europe and
using different types of computers and software
WWW
Berners-Lee's design of the Web integrated two key
ideas:
 Hypertext (documents with interlinked text and
media--images and links to other pages
 the distributed nature of the Internet
 pages can be stored on machines all across the
Internet, known as Web servers
World Wide Web (WWW)
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Like Internet communications, the Web relies on
protocols to ensure that pages are accessible to
any computer
 HyperText Transfer Protocol (HTTP) defines
how messages are exchanged between
browsers and servers (to be defined).
 The prefix http:// in a URL specifies that the
HTTP protocol is to be used in communicating
with the server.
Net and Web
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The World Wide Web is a collection of software that
spans the Internet and enables the interlinking of
documents and resources
WWW uses the HTTP protocol.
Many different services run “over” the Internet
Services such as FTP, Telnet and email were
established long before the WWW and can be used
independently of the WWW
Internet ≠ World Wide Web
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The Internet could exist without the Web and did, in
fact, for many years
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The Web couldn't exist without the Internet
 WWW runs over the internet.
 The Internet is the underlying communication
network that executes the Web software
Web Page
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A Web page is a text document that contains
additional formatting information in a language
called HTML (HyperText Markup Language) which
we will learn.
A Web browser is a program that fetches an
HTML file, interprets its content, and displays the
page the way we like to see it. e.g. Internet
Explorer
Web Server
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A Web server is an Internet-enabled computer
that stores Web pages and executes software for
providing access to the pages
 when you request a Web page, the browser
sends a request over the Internet to the server
 the server locates the specified page and sends
it back to your computer
 The browser interprets and displays the page
Web Addresses
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Web pages require uniform names to locate and
identify them uniquely
 each page is assigned a Uniform Resource
Locator (URL)
 URL's are commonly referred to as Web
addresses
 the different parts of the Web address provide
information for locating the page
URL - uniform resource locator
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http://www.sci.brooklyn.cuny.edu/~co
gan
First part - http:// - specifies the
protocol to be used when retrieving
this resource. Web browsers fill it in,
because http is the default.
URL
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Second part - DNS name of server www.sci.brooklyn.cuny.edu
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Third part - name or path to the file on the
server.
~cogan
URL
Viewing Local Web Pages
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a Web browser can be used to view pages stored on
the same computer
 can go through the File menu to select the local
page, or
 can enter the File location in the address box
(without the http prefix)
this feature is handy when developing Web pages
 can create a Web page and view it in the browser
before uploading to a server
Email address
An email address is
Account name @ DNS name
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