Transcript Network Layer (Continued)

Introduction To Networking
What is a computer Network
• Definition:
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Types of Networks
• Networks by components
– P2P
- Server-Based
• Networks by Size
– LAN
- MAN
- WAN
• Networks by Topology
– Star
- Bus
- Ring
- Hierarchy -Mesh
• Networks by Media
– Guided
- Unguided
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Network Standards
• A number of organizations promote
standardized components of data
communications
• Examples:
– ISOC
– ANSI
– ITU
– IEEE
– ISO
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Understanding the OSI Model
• In an effort to identify and standardize all
the levels of communication needed in
networking, ISO developed a networking
model called the Open Systems
Interconnection (OSI) reference model.
• The OSI reference model provides a
universally accepted reference illustrating
how data is transmitted on a network or
between two or more networked devices.
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OSI Reference Model
• Foundation that brings continuity to LAN
and WAN communications
• Product of two standards organizations:
– ISO
– ANSI
• Developed in 1974
• Set of communication guidelines for
hardware and software design
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OSI Guidelines Specify…
• Methods to ensure that network
transmissions are received correctly
• How network devices maintain a
consistent rate of data flow
• How electronic data is represented on
network media
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OSI Layers
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Understanding the OSI Model
(Continued)
Application Layer
• The Application layer of the OSI model is
responsible for interfacing with application
software, such as Web browsers or Web
servers.
Presentation Layer
• The Presentation layer receives requests for
files from the Application layer, and presents the
requests to the Session layer.
• The Presentation layer reformats, compresses,
or encrypts data as necessary.
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Understanding the OSI Model
(Continued)
Session Layer
• The Session layer is responsible for
establishing and maintaining a session
between two networked stations or hosts.
• A host is any computer or other device on
a network that has been assigned an IP
address.
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Understanding the OSI Model
(Continued)
Transport Layer
• The Transport layer is responsible for error
checking and requests retransmission of
data if it detects errors.
• The Transport layer might or might not
guarantee successful delivery of data.
Network Layer
• The Network layer is responsible for
dividing a block of data into segments that
are small enough to travel over a network.
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Understanding the OSI Model
(Continued)
Network Layer (Continued)
• Segments of data are called packets, data
packets, or datagrams and contain data, along
with special identifying information in headers
and trailers at the beginning and end of the
packet.
Data Link Layer
• The Data Link layer is responsible for receiving
packets of data from the Network layer and
presenting them to the Physical layer for
transport.
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Understanding the OSI Model
(Continued)
Physical Layer
• The OSI Physical layer is responsible for
passing data packets on to the cabling or
wireless media (whether the media be
cabling or wireless).
• When software is permanently embedded
on a hardware device, it is called
firmware.
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Communicating Between
Stacks
• OSI model provides standards for:
– Communicating on a LAN
– Communicating between LANs
– Internetworking between LANs and WANs
and between WANs and WANs
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Peer Protocols
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Applying the OSI Model
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The OSI Model Applied to a
TCP/IP Network
• On a TCP/IP network, TCP/IP is managed
by the operating system and covers the
first five layers of the OSI model.
• When a browser wants to access a Web
server, it uses the address and port of the
Web server to make a request to the
operating system.
• This request takes the form of an
Application Program Interface (API).
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The OSI Model Applied to a
TCP/IP Network (Continued)
• In a TCP/IP network, the API call for a
Web page from a Web server causes the
operating system to generate an HTTP
request.
• HTTP does not operate in the lower layers
of the network model.
• Instead, it hands data over to TCP, which
resides in the Transport layer.
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The OSI Model Applied to a
TCP/IP Network (Continued)
• A frame provides information at the beginning of
the data, called a header, and information at the
end of the data, called a trailer.
• A checksum is a calculated value that can be
compared to the value calculated by the
receiving Data Link layer.
• This technique of calculating and comparing
values is called a cyclical redundancy check
(CRC).
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TCP/IP Protocols at Each Layer
• TCP/IP covers the first five layers of the
OSI model, and is included in an operating
system as a group of utilities called the
TCP/IP stack.
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Protocols at the Application,
Presentation, and Session Layers
• The first three layers of the OSI model are
handled by the protocol specific to the
application using it and are best treated as a
single group rather than unique layers.
• Web browsers, e-mail, chat rooms, and FTP
software are examples of the applications that
use the Internet.
• The language or protocol each of these
applications uses is listed at the Application,
Presentation, and Session layers.
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Protocols at the Transport Layer
• A TCP/IP network has two protocols that
work at the Transport layer; one protocol
guarantees delivery and the other does
not.
• With TCP/IP, the protocol that guarantees
delivery is TCP and the protocol that does
not is UDP (User Datagram Protocol).
• TCP is used for client and server requests
and responses.
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Protocols at the Transport Layer
(Continued)
• Because TCP establishes a connection, it
is called a connection-oriented protocol.
• UDP is a protocol that sends data without
caring about whether the data is received.
• It does not establish a connection first;
thus, it is called a connectionless
protocol.
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Protocols at the Network Layer
• TCP and UDP communicate with the Network
layer, which is sometimes called the Internet layer.
• Some of the other supporting protocols include
ARP (Address Resolution Protocol),
responsible for locating a host on a LAN; RARP
(Reverse Address Resolution Protocol),
responsible for discovering the Internet address of
a host on a LAN; and ICMP (Internet Control
Message Protocol), responsible for
communicating problems with transmission to
devices that need to know about these problems.
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Protocols at the Data Link and
Physical Layers
• PPP (Point-to-Point Protocol) is used
over telephone lines, and allows a
computer to connect to a network using a
modem.
• PPP is the most popular protocol for
managing network transmission from one
modem to another.
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MAC Addresses
• MAC addresses function at the lowest
(Data Link) networking level.
• If a host does not know the MAC address
of another host on a local area network, it
uses the operating system to discover the
MAC address.
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IP Addresses
• All the protocols of the TCP/IP suite
identify a device on the Internet or an
intranet by its IP address.
• An IP address is 32 bits long, made up of
4 bytes separated by periods.
• Within an IP address, each of the four
numbers separated by periods is called an
octet.
• The first part of an IP address identifies
the network, and the last part identifies the
host.
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Classes of IP Addresses
• IP addresses that can be used by companies
and individuals are divided into three classes:
Class A, Class B, and Class C, based on the
number of possible IP addresses in each
network within each class.
• The group of IP addresses assigned to an
organization are unique to all other IP addresses
on the Internet and are available for use on the
Internet.
• The IP addresses available to the Internet are
called public IP addresses.
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Private IP Addresses
• Private IP addresses are IP addresses
that are assigned by a network
administrator for use on private intranets
that are isolated from the Internet.
• The RFC 1918 recommends that the
following IP addresses be used for private
networks:
– 10.0.0.0 through 10.255.255.255
– 172.16.0.0 through 172.31.255.255
– 192.168.0.0 through 192. 168. 255.255
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Dynamically Assigned IP
Addresses
• Instead of IP addresses permanently being
assigned to computers (called static IP
addresses), an IP address is assigned for
the current session only (called a dynamic
IP address).
• Internet service providers (ISPs) are
organizations through which individuals
and businesses connect to the Internet.
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Network Address Translation
• If the hosts on a network using private IP
addresses need to access the Internet, a
problem arises because the private IP
addresses are not allowed on the Internet.
• The solution is to use NAT (Network
Address Translation), which uses a
single public IP address to access the
Internet on behalf of all hosts on the
network using other IP addresses.
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Plans for New IP Addresses
• Because of an impending shortage of IP
addresses, as well as some limitations in
the current standards for IP, a new
scheme of IP addresses called the IPv6
(IP version 6) standard is currently being
developed and implemented.
• Current IP addresses using the current
IPv4 (IP version 4) have 32 bits with eight
bits in each of four octets.
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Plans for New IP Addresses
(Continued)
• With the new system, each address
segment can have 32 bits, for a total of
128 bits for the entire address.
• A disadvantage of IPv6 is the fact that so
much software used on the Internet would
become outdated because current
software is designed to hold 32-bit IP
addresses and, with the new system, this
number would no longer be sufficient.
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Ports
• A port is a number used to address
software or services running on a
computer.
• A host computer might have several
services running on it.
• Each server running on the host is
assigned a port.
• The port is written at the end of the IP
address, separated from the IP address
with a colon—like this: 169.49.209.19:80.
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Tying It All Together: How Data
Travels on the Network
• The Web browser wants to make a
request to a Web server, and processes
the request using an API call.
• The API process packages the data using
HTTP format, which includes an HTTP
header, and addresses it to an IP address
and port 80, which is the default port for a
Web server.
• HTTP delivers the package to TCP, giving
the destination IP address and port.
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Tying It All Together: How Data
Travels on the Network (Continued)
• TCP guarantees delivery, and attempts to make
a connection to the destination IP address and
port using the three-way handshake.
• With the first attempt that TCP makes to
communicate with the remote host, the first
packet reaches the Network layer and IP uses
ARP to resolve the IP address to a MAC
address.
• When a packet reaches a hub on the Ethernet
LAN, the hub replicates the packet and sends it
to every device to which it is connected.
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Tying It All Together: How Data
Travels on the Network (Continued)
• When the IP layer receives a broadcast
packet, it decides if it is appropriate to
respond.
• After it knows the MAC address, it can
send the first packet to make the
connection.
• It prepares the data by attaching a TCP
header in front of the data.
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