Download Introduction to the Internet, ping and traceroute

Get the Net!
Introduction to the Internet, World
Wide Web, and the Utilities of Ping
and Traceroute
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Networks
• A network is a collection of computers
connected together so that they can share
information.
• A network is called a local area network (or
LAN) if the constituent computers are all
located in the same vicinity.
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Other networks
• Metropolitan area networks (MANs) are
spread throughout a city or county. For
example, a cellular phone network.
• Wide area networks (WANs) are spread
over an even wider area, a telephone
company, for instance.
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Internet
• When two or more networks are connected,
they form an internet. (small i)
• The Internet (capital I) is the global
collection of connected networks.
• “The biggest WAN of them all”
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A Little History
• An early ancestor of the Internet was called
ARPANET (ARPA stands for Advanced
Research Project Agency).
• It was built in the late 60’s by a
collaboration of universities and the
military.
• One desired feature was a network that
would still function if part of it was
“knocked out.”
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Connections
• The whole point of being “connected”
is that information can be passed back
and forth.
– Originally it was more about sharing
processing power which was scarce, then
as processors became more powerful it
was more about sharing information.
• Information (signals) can be sent
through the air (as in radio, non-cable
television or wireless networks) or
through wires (as in telephone, cable
tv or wired networks).
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Wire
• Twisted pair wire looks somewhat like the wire
to your telephone.
• Coaxial cable looks like the wire to your cable
tv.
• Fiber optic cable sends light beams.
• Each successive wire has more bandwidth;
bandwidth is related to the rate of information
that can be send.
• (bandwidth  wire thickness)
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Packets
• On the Internet, the information is broken
into pieces, called packets, which are sent
individually over the wire.
• The packets must also contain the
information that allows the computer to
reassemble them.
• More like sending letters than talking on the
phone.
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Routers
• Packets are to letters as routers are to sorters
in the post office.
• A router is the intelligent part of the
connection that directs the information to
the right place.
• The analog of one’s address and zip code is
the Internet protocol (IP) address.
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Inter Vs. Intra
• Inter: between or among
• Intra: within or inside of
• Intranet: a set-up like the Internet (having
browsers, email, etc.) but not connected to
the outside world (often used by companies
wanting to maintain some privacy).
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Firewalls
• A firewall restricts the flow of information both
into and out of a network.
• An attempt to have the best of both worlds:
connectivity and access to information on one
hand, privacy and security on the other.
• In the mail analogy, it is as though your mail is
being censored.
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The Web  The Net
• The World Wide Web is only part of the
Internet.
• The Internet also includes
– e-mail (electronic mail): to send messages to
and receive messages from, on the same or
other networks
– ftp (file transfer protocol): to put or get files
from other computers
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The Net > The Web
• The Internet also incudes:
– telnet: to log onto a computer that one is not
physically in front of
– gopher: to find files on the Internet; gopher is a
precursor to a “search engine”
– usenet: to promote discussions on various
topics among discussion or news groups
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The World Wide Web
• The Web is a hyperlinked multimedia
database.
• It is a vast collection of information stored
in files (hence a database, at least in an
informal sense).
• It appears as documents with text, as well as
graphics, audio, animation, and video
(hence multimedia).
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The Web
• Finally, the documents (web pages) are
connected to one another via hyperlinks, a
reference to another web page which if
clicked takes one to that page.
• Moving about in this fashion is generally
referred to as “surfing.”
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Web Vs. Tree
• Gopher is older than the Web and also
allows one to access files.
• It has a “hierarchical” or tree structure.
• One could not jump from one branch to
another; one had to climb back down the
tree.
• The Web, if less structured, is more highly
connected.
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Tree (like
Windows
Explorer)
Web
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Getting Around (URL’s)
• The fancy term for a web site address is a
uniform resource location (URL)
• A URL consists of several parts, e.g.
• http://www.lasalle.edu/lsu-site/it/notes.htm#attach
– protocol: a set of standards allowing computers
to exchange information
– http - “hypertext transfer protocol” (others: ftp,
gopher, telnet, news)
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URL’s (cont.)
• http://www.lasalle.edu/lsu-site/it/notes.htm#attach
– domain: denotes the computer that holds the
web page (stands in for the IP address)
– often starts with www (World Wide Web)
– ends with the type of organization operating the
site or the country (known as the top-level
domain)
• edu (education), gov (government), mil (military),
org (organization), net (networks), uk (United
Kingdom), ch (Switzerland)
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URL’s (cont.)
• http://www.lasalle.edu/lsu-site/it/notes.htm#attach
– the folder (or directory) containing the file,
also known as the path
• http://www.lasalle.edu/lsu-site/it/notes.htm#attach
– the file or document (often an html file)
• http://www.lasalle.edu/lsu-site/it/notes.htm#attach
– anchor or fragment, indicates a specific part of
a document
• (A URL can also have a port number)
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Ping and traceroute
• Having a network is about being connected –
having information be directed from one
computer to another.
• One can check on connectivity using ping and
actually determine the path information takes
using traceroute.
• Material based on Chapter 2 in Computer
Networks and Internets (Comer)
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Pinging
• Some say ping is short for Packet Internet
Groper.
• It is a utility that sends a message to a computer
and waits for a response.
– A utility is part of system software that is used to help
manage the computer’s resources, as opposed to
application software.
• If the response is received, information about the
time required and so on are reported.
• If the response is not received in a certain amount
of time, the request is said to have “timed out.”
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Pinging (Cont.)
• It works by sending special packets to a
destination – an IP address or domain name.
– The special packets belong to Internet Control
Message Protocol (ICMP).
– You send a echo request and you receive an
echo reply.
• If the destination responds, you have a good
TCP/IP connection (working route).
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Breaking it down
• Pinging consists of the command “ping”
followed by an IP address or domain name
– ping 139.84.10.250
– ping www.lasalle.edu
• It sends out special packets which request a
response (echo reply) if the destination host
is reached.
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Host
• A host is a computer that is connected to a
TCP/IP network, including the Internet.
• Each host has a unique IP address.
– A computer may always have the same address
or may have a different address each time it
connects to the networks.
– Two computers on the same network or internet
at the same time should not have the same
address.
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IP (v4) Address
• A 32-bit number that identifies a computer or
device on a TCP/IP network.
– TCP/IP stands for Transmission Control
Protocol/Internet Protocol
• A 32-bit number consists of four bytes
– A byte is eight bits (a.k.a. an octet)
– 256 (0 through 255) numbers can be represented using
a byte
– 256 = 28 = 2  2  2  2  2  2  2  2
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Numbers represented using a
byte
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
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ETC., ETC.
00001000
00001001
00001010
00001011
00001100
00001101
00001110
00001111
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Usual form
• Instead of writing out a string of thirty-two
1’s and 0’s, IP addresses are usually written
as four numbers between 0 and 255
separated by periods (the dotted decimal
notation)
• For example
139.84.10.250
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How many?
• With 32 bits, 232 devices could be
distinguished.
• Actually the number of valid IP addresses is
less than 232 because certain addresses are
reserved for special purposes (127.0.0.1 is
one example we’ll see later)
– Another problem is that addresses are “owned”
by people not using them.
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Running out
• IP addresses are broken into classes (more on this
later in the semester)
– Class A - supports 16 million hosts on each of 127
networks.
– Class B - supports 65,000 hosts on each of 16,000
networks.
– Class C - supports 254 hosts on each of 2 million
networks.
• The number of unassigned Internet addresses is
running out, so a new classless scheme called
CIDR is gradually replacing the system based on
classes A, B, and C and is tied to adoption of IPv6.
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The Next Generation
• IPv6 (IP version 6), a.k.a. IPng: Internet
Protocol next generation is an extended
version of the Internet Protocol (IP)
“Recommended” by an IETF standards
committee.
• The commonly used version of IP is version
4, so it is sometimes referred to as IPv4.
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IP Address (cont.)
• In an isolated network, any set of IP
addresses can be assigned.
• On the other hand, connecting a network to
the Internet requires using registered IP
addresses to avoid duplications.
• The InterNIC Registration Service assigns
Internet addresses.
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Domain name
• A domain name is a string of characters standing
in for an IP address.
• It can be more complicated, it can be that
– One Name  Multiple Addresses, or
– One Address  Multiple Names
• Domain names are used in URLs to identify
particular Web pages. For example, in the URL
http://www.pcwebopedia.com/index.html, the
domain name is pcwebopedia.com.
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Packet
• A packet is a message (protocol + “some”
data) or a piece of a message transmitted
over a packet-switching network.
• A packet must also contain an address
(identifier) so that it can be routed to its
destination.
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Packet switching
• A set of protocols (rules for communication) in
which messages are broken into pieces called
packets before being sent.
• The packets are transmitted individually
(connectionless).
– They may follow different routes.
– They can arrive out of order.
– Share the medium with other packets from other
transmissions.
• When the packets arrive, they are reassembled into
the original message.
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As opposed to Circuit switching
• Another way to send information is called
circuit switching (connection oriented).
• A telephone call is an example of circuit
switching. A dedicated line is allocated for
transmission between two parties.
• Preferred when speed and the order the data
is sent in are important, as is the case with
real-time data, such as live audio and video.
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Pinging: Start/Run
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Pinging: Type command in Run
dialog box
Older systems might use command instead
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Pinging: type ping followed by
domain name
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Pinging a host which was off at
the time
Timed out
Can use IP instead of domain name
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Pinging webopedia, also timed
out
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Webopedia ping timed out, even
though site was up
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Denial of service
• One possibility is that some companies
block pings because their systems could
become clogged with ping requests.
• Rejecting pings could make a site less
vulnerable to a Denial of Service (DoS)
attack, in which a site is flooded with so
many requests that it cannot answer them
all.
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Slow connection versus no
connection
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The network was slow when I was doing this.
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Type ping /? To find the various
options
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Using the count request option to
send 5 instead of 4
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Useful or not?
• Simply pinging a remote host (a computer
that is on one’s internet but not on one’s
network) usually has an all or nothing
outcome and as such can provide somewhat
limited information.
• A sequence of pinging can provide more
information (see MCSE TCP/IP for
Dummies).
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Pinging yourself using localhost or
127.0.0.1: verifies that TCP/IP has
been installed and started correctly
Reserved IP address
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Pinging yourself using your IP,
checks that the IP is valid
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You can use another utility ipconfig
to find your IP if you don’t know it
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Pinging the Gateway – timing out here
may indicate a problem with the router
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Gateway
• Recall that one gets an internet by
connecting more networks.
• The point at which data leaves one (local)
network and enters another is known as a
gateway.
• A node (a device on the Internet) can be
either a gateway, router, or a host (endpoint).
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Host/Gateway
• When you are looking at some site on the
net, your computer and the one containing
the site are hosts.
• On the other hand, the computers that direct
the traffic, for example your Internet
service provider (ISP) are gateway/router
nodes.
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Traceroute
• A utility that traces (determines the route taken by)
a packet from your computer (the local host) to
some other host on the network.
– It doesn’t actually follow a single packet but sends out a
series of packets.
– The packets are restricted in the number of “hops” they
can take.
• It is limited by the time-to-live (TTL) field
– When the limit is reached, information about the node
is (should be) returned.
– Each consecutive packet is allowed one more hop.
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Hop
• A hop is when a packet is transmitted from
one node to the next.
• “Some Internet Service Providers (ISPs)
advertise how many hops away from
Internet backbone they are. Theoretically,
the fewer hops it takes to get your data onto
the backbone, the faster your access will
be.” (webopedia)
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Traceroute (Cont.)
• Traceroute shows how many hops were
required to reach the destination as well as
the length of time for each hop.
• When the web is sluggish, you can use
traceroute to determine where the
congestion lies.
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Traceroute (Cont.)
• The original traceroute was for UNIX utility
• Windows has a traceroute utility called
tracert.
• Go to Start/Run, type cmd, and then type
“tracert” followed by the domain name of
the host.
• For example:
tracert www.webopedia.com
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tracert www.whatis.com
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Other TCP/IP diagnostic
commands
•
•
•
•
•
•
Nbtstat
Netstat
Ipconfig
Arp
Hostname
route
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Other sources
• http://www.webopedia.com
• http://www.whatis.com
• MCSE TCP/IP for Dummies (Cameron
Brandon)
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