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THE OSI MODEL
Application
Presentation
Session
Transport
Network
Data-Link
Physical
Semesters 1 & 2
Concept Review
Chapter 1—Review
By: Allan Johnson
Table of Contents
Go There!
• Review the OSI Model
Go There!
• LAN Devices & Technologies
Go There!
• IP Addressing
Go There!
• CIDR Notation
Go There!
• Routing
Go There!
• Transport Layer
Arhitectura generala a Internetului
Arhitectura Internet - 2
Componente Internet
Protocol de retea - definitie
• un protocol defineşte formatul şi ordinea mesajelor schimbate
între două sau mai multe entităţi ce comunică între ele, precum
şi acţiunile ce sunt întreprinse odată cu transmiterea sau recepţia
unui mesaj sau a unui alt eveniment.
THE OSI MODEL
Application
Presentation
Review The Model
Session
Transport
Network
Data-Link
Physical
Open Systems
Interconnected Reference
Model
Table of Contents
Why A Layered Model?
Application
Presentation
Session
Transport
Network
Data-Link
Physical
• Reduces complexity
• Standardizes interfaces
• Facilitates modular
engineering
• Ensures interoperable
technology
• Accelerates evolution
• Simplifies teaching &
learning
Application Layer
Application
Presentation
Session
Transport
Network
Data-Link
Physical
 Provides network services
(processes) to applications.
 For example, a computer on
a LAN can save files to a
server using a network
redirector supplied by NOSs
like Novell.
 Network redirectors allow
applications like Word and
Excel to “see” the network.
Presentation Layer
Application
Presentation
Session
Transport
Network
Data-Link
Physical
 Provides data representation
and code formatting.
 Code formatting includes
compression and encryption
 Basically, the presentation
layer is responsible for
representing data so that
the source and destination
can communicate at the
application layer.
Session Layer
Application
Presentation
Session
Transport
Network
Data-Link
Physical
 Provides inter-host
communication by establishing,
maintaining, and terminating
sessions.
 Session uses dialog control and
dialog separation to manage the
session
 Some Session protocols:






NFS (Network File System)
SQL (Structured Query Language)
RCP (Remote Call Procedure)
ASP (AppleTalk Session Protocol)
SCP (Session Control Protocol)
X-window
Transport Layer
Application
Presentation
Session
Transport
Network
Data-Link
Physical
 Provides reliability, flow control,
and error correction through the
use of TCP.
 TCP segments the data, adding a
header with control information
for sequencing and
acknowledging packets received.
 The segment header also
includes source and destination
ports for upper-layer applications
 TCP is connection-oriented and
uses windowing.
 UDP is connectionless. UDP does
not acknowledge the receipt of
packets.
Network Layer
Application
Presentation
Session
Transport
Network
Data-Link
Physical
 Responsible for logically
addressing the packet and
path determination.
 Addressing is done through
routed protocols such as IP,
IPX, AppleTalk, and DECnet.
 Path Selection is done by
using routing protocols such
as RIP, IGRP, EIGRP, OSPF,
and BGP.
 Routers operate at the
Network Layer
Data-Link Layer
Application
Presentation
Session
Transport
Network
Data-Link
Physical
 Provides access to the media
 Handles error notification,
network topology issues, and
physically addressing the
frame.
 Media Access Control through
either...
 Deterministic—token passing
 Non-deterministic—broadcast
topology (collision domains)
 Important concept: CSMA/CD
Physical Layer
Application
Presentation
Session
Transport
Network
Data-Link
Physical
 Provides electrical,
mechanical, procedural and
functional means for
activating and maintaining
links between systems.
 Includes the medium through
which bits flow. Media can
be...




CAT 5 cable
Coaxial cable
Fiber Optics cable
The atmosphere
Peer-to-Peer Communications
• Peers communicate using the PDU of their
layer. For example, the network layers of the
source and destination are peers and use
packets to communicate with each other.
Application
Data
Application
Presentation
Data
Presentation
Session
Session
Transport
Data
Segments
Transport
Network
Packets
Network
Data-Link
Frames
Data-Link
Physical
Bits
Physical
Encapsulation Example
Application
Presentation
Session
Transport
Network
Data-Link
Physical
• You type an email
message. SMTP takes the
data and passes it to the
Presentation Layer.
• Presentation codes the
data as ASCII.
• Session establishes a
connection with the
destination for the purpose
of transporting the data.
Encapsulation Example
Application
Presentation
Session
Transport
Network
Data-Link
Physical
• Transport segments the
data using TCP and hands it
to the Network Layer for
addressing
• Network addresses the
packet using IP.
• Data-Link then encaps. the
packet in a frame and
addresses it for local
delivery (MACs)
• The Physical layer sends the
bits down the wire.
THE OSI MODEL
Application
Presentation
Session
Transport
Network
Data-Link
LAN Devices &
Technologies
The Data-Link &
Physical Layers
Physical
Table of Contents
Devices
What layer device?
• What does it do?
 Connects LAN
segments;
 Filters traffic based
on MAC addresses;
and
 Separates collision
domains based upon
MAC addresses.
Devices
• What does it do?
What layer device?
 Since it is a multiport bridge, it can
also
 Connect LAN
segments;
 Filter traffic based on
MAC addresses; and
 Separate collision
domains
 However, switches
also offer full-duplex,
dedicated bandwidth
to segments or
desktops.
Devices
What layer device?
• What does it do?
 Concentrates LAN
connections from
multiple devices into
one location
 Repeats the signal (a
hub is a multi-port
repeater)
Devices
• What does it do?
What layer device?
 Interconnects networks
and provides broadcast
control
 Determines the path
using a routing protocol
or static route
 Re-encapsulates the
packet in the appropriate
frame format and
switches it out the
interface
 Uses logical addressing
(i.e. IP addresses) to
determine the path
Media Types
LAN Technologies
Three Most
Common Used
Today in
Networking
Ethernet/802.3
• Cable Specifications:
 10Base2
 Called Thinnet; uses coax
 Max. distance = 185 meters (almost 200)
 10Base5
 Called Thicknet; uses coax
 Max. distance = 500 meters
 10BaseT
 Uses Twisted-pair
 Max. distance = 100 meters
 10 means 10 Mbps
Ethernet/802.3
• Ethernet is broadcast topology.
 What does that mean?
 Every devices on the Ethernet segment sees
every frame.
 Frames are addressed with source and
destination ______ addresses.
 When a source does not know the destination
or wants to communicate with every device, it
encapsulates the frame with a broadcast MAC
address: FFFF.FFFF.FFFF
 What is the main network traffic problem
caused by Ethernet broadcast topologies?
Ethernet/802.3
• Ethernet topologies are also shared
media.
• That means media access is controlled
on a “first come, first serve” basis.
• This results in collisions between the
data of two simultaneously transmitting
devices.
• Collisions are resolved using what
method?
Ethernet/802.3
• CSMA/CD (Carrier Sense Multiple Access with
Collision Detection)
• Describe how CSMA/CD works:
 A node needing to transmit listens for activity on
the media. If there is none, it transmits.
 The node continues to listen. A collision is
detected by a spike in voltage (a bit can only be a
0 or a 1--it cannot be a 2)
 The node generates a jam signal to tell all devices
to stop transmitting for a random amount of time
(back-off algorithm).
 When media is clear of any transmissions, the
node can attempt to retransmit.
Address Resolution Protocol
• In broadcast topologies, we need a way to
resolve unknown destination MAC addresses.
• ARP is protocol where the sending device
sends out a broadcast ARP request which
says, “What’s you MAC address?”
• If the destination exists on the same LAN
segment as the source, then the destination
replies with its MAC address.
• However, if the destination and source are
separated by a router, the router will not
forward the broadcast (an important function
of routers). Instead the router replies with its
own MAC address.
THE OSI MODEL
Application
Presentation
IP Addressing
Session
Transport
Network
Subnetting Review
Data-Link
Physical
Table of Contents
Logical Addressing
• At the network layer, we use logical,
hierarchical addressing.
• With Internet Protocol (IP), this address is a
32-bit addressing scheme divided into four
octets.
• Do you remember the classes 1st octet’s
value?





Class
Class
Class
Class
Class
A: 1 - 126
B: 128 - 191
C: 192 - 223
D: 224 - 239 (multicasting)
E: 240 - 255 (experimental)
Network vs. Host
Class A:
27 = 126 networks; 224 > 16 million hosts
N
Class B :
H
H
214 = 16,384 networks; 216 > 65,534 hosts
N
Class C :
H
N
H
H
221 > 2 million networks; 28 = 254 hosts
N
N
N
H
Why Subnet?
• Remember: we are usually dealing with
a broadcast topology.
• Can you imagine what the network
traffic overhead would be like on a
network with 254 hosts trying to
discover each others MAC addresses?
• Subnetting allows us to segment LANs
into logical broadcast domains called
subnets, thereby improving network
performance.
Four Subnetting Steps
• To correctly subnet a given network
address into subnet addresses, ask
yourself the following questions:
1.
2.
3.
4.
How many bits do I need to borrow?
What’s the subnet mask?
What’s the “magic number” or multiplier?
What are the first three subnetwork
addresses?
• Let’s look at each of these questions in
detail
1. How many bits to borrow?
• First, you need to know how many bits
you have to work with.
• Second, you must know either how
many subnets you need or how many
hosts per subnet you need.
• Finally, you need to figure out the
number of bits to borrow.
1. How many bits to borrow?
• How many bits do I have to work with?
 Depends on the class of your network
address.
 Class C: 8 host bits
 Class B: 16 host bits
 Class A: 24 host bits
 Remember: you must borrow at least 2 bits
for subnets and leave at least 2 bits for
host addresses.
2
 2 bits borrowed allows 2 - 2 = 2 subnets
1. How many bits to borrow?
• How many subnets or hosts do I need?
• A simple formula:
 Total Bits = Bits Borrowed + Bits Left
 TB = BB + BL
• I need x subnets:
• I need x hosts:
2 2x
BL
2 2x
BB
• Remember: we need to subtract two to
provide for the subnetwork and
broadcast addresses.
1. How many bits to borrow?
• Class C Example: 210.93.45.0
• Design goals specify at least 5 subnets
so how many bits do we borrow?
• How many bits in the host portion do
we have to work with (TB)?
• What’s the BB in our TB = BB + BL
formula? (8 = BB + BL)
• 2 to the what power will give us at least
5 subnets?
3
2 - 2 = 6 subnets
1. How many bits to borrow?
• How many bits are left for hosts?
TB = BB + BL
8 = 3 + BL
BL = 5
• So how many hosts can we assign to
each subnet?
5
2 - 2 = 30 hosts
1. How many bits to borrow?
• Class B Example: 185.75.0.0
• Design goals specify no more than 126 hosts
per subnet, so how many bits do we need to
leave (BL)?
• How many bits in the host portion do we have
to work with (TB)?
• What’s the BL in our TB = BB + BL formula?
(16 = BB + BL)
• 2 to the what power will insure no more than
126 hosts per subnet and give us the most
subnets?
7
2 - 2 = 126 hosts
1. How many bits to borrow?
• How many bits are left for subnets?
TB = BB + BL
16 = BB + 7
BL = 9
• So how many subnets can we have?
9
2 - 2 = 510 subnets
2. What’s the subnet mask?
• We determine the subnet mask by adding up
the decimal value of the bits we borrowed.
• In the previous Class C example, we borrowed
3 bits. Below is the host octet showing the
bits we borrowed and their decimal values.
1
1
1
128
64
32
16
8
4
2
1
We add up the decimal value of these bits and get 224.
That’s the last non-zero octet of our subnet mask.
So our subnet mask is 255.255.255.224
3. What’s the “magic number?”
• To find the “magic number” or the
multiplier we will use to determine the
subnetwork addresses, we subtract the
last non-zero octet from 256.
• In our Class C example, our subnet
mask was 255.255.255.224. 224 is our
last non-zero octet.
• Our magic number is 256 - 224 = 32
Last Non-Zero Octet
• Memorize this table. You should be able to:
 Quickly calculate the last non-zero octet when
given the number of bits borrowed.
 Determine the number of bits borrowed given the
last non-zero octet.
 Determine the amount of bits left over for hosts
and the number of host addresses available.
Bits
Non-Zero
Borrowed Octet
Hosts
2
192
62
3
224
30
4
240
14
5
248
6
6
252
2
4. What are the subnets?
• We now take our “magic number” and
use it as a multiplier.
• Our Class C address was 210.93.45.0.
• We borrowed bits in the fourth octet, so
that’s where our multiplier occurs
 1st subnet: 210.93.45.32
 2nd subnet: 210.93.45.64
 3rd subnet: 210.93.45.96
• We keep adding 32 in the fourth octet
to get all six available subnet addresses.
Host & Broadcast Addresses
• Now you can see why we subtract 2 when
determining the number of host address.
• Let’s look at our 1st subnet: 210.93.45.32
• What is the total range of addresses up to our
next subnet, 210.93.45.64?
• 210.93.45.32 to 210.93.45.63 or 32 addresses
• .32 cannot be assigned to a host. Why?
• .63 cannot be assigned to a host. Why?
• So our host addresses are .33 - .62 or 30 host
addresses--just like we figured out earlier.
THE OSI MODEL
Application
Presentation
CIDR Notation
Session
Transport
Network
Data-Link
A Different Way to
Represent a Subnet Mask
Physical
Table of Contents
CIDR Notation
• Classless Interdomain Routing is a method of
representing an IP address and its subnet
mask with a prefix.
• For example: 192.168.50.0/27
• What do you think the 27 tells you?
 27 is the number of 1 bits in the subnet mask.
Therefore, 255.255.255.224
 Also, you know 192 is a Class C, so we borrowed 3
bits!!
 Finally, you know the magic number is 256 - 224 =
32, so the first useable subnet address is
197.168.50.32!!
• Let’s see the power of CIDR notation.
202.151.37.0/26
• Subnet mask?
 255.255.255.192
• Bits borrowed?
 Class C so 2 bits borrowed
• Magic Number?
 256 - 192 = 64
• First useable subnet address?
 202.151.37.64
• Third useable subnet address?
 64 + 64 + 64 = 192, so 202.151.37.192
198.53.67.0/30
• Subnet mask?
 255.255.255.252
• Bits borrowed?
 Class C so 6 bits borrowed
• Magic Number?
 256 - 252 = 4
• Third useable subnet address?
 4 + 4 + 4 = 12, so 198.53.67.12
• Second subnet’s broadcast address?
 4 + 4 + 4 - 1 = 11, so 198.53.67.11
200.39.89.0/28
• What kind of address is 200.39.89.0?




Class C, so 4 bits borrowed
Last non-zero octet is 240
Magic number is 256 - 240 = 16
32 is a multiple of 16 so 200.39.89.32 is a
subnet address--the second subnet
address!!
• What’s the broadcast address of
200.39.89.32?
 32 + 16 -1 = 47, so 200.39.89.47
194.53.45.0/29
• What kind of address is 194.53.45.26?





Class C, so 5 bits borrowed
Last non-zero octet is 248
Magic number is 256 - 248 = 8
Subnets are .8, .16, .24, .32, ect.
So 194.53.45.26 belongs to the third subnet
address (194.53.45.24) and is a host address.
• What broadcast address would this host use
to communicate with other devices on the
same subnet?
 It belongs to .24 and the next is .32, so 1 less is
.31 (194.53.45.31)
No Worksheet Needed!
• After some practice, you should never need a
subnetting worksheet again.
• The only information you need is the IP
address and the CIDR notation.
• For example, the address 221.39.50/26
• You can quickly determine that the first
subnet address is 221.39.50.64. How?
 Class C, 2 bits borrowed
 256 - 192 = 64, so 221.39.50.64
• For the rest of the addresses, just do
multiples of 64 (.64, .128, .192).
The Key!!
• MEMORIZE THIS TABLE!!!
Bits
Non-Zero
Borrowed Octet
Hosts
2
192
62
3
224
30
4
240
14
5
248
6
6
252
2
Practice On Your Own
•
Below are some practice problems. Take out
a sheet of paper and calculate...



1.
2.
3.
4.
5.
6.
7.
Bits borrowed
Last non-zero octet
Second subnet address and broadcast address
192.168.15.0/26
220.75.32.0/30
200.39.79.0/29
195.50.120.0/27
202.139.67.0/28
Challenge: 132.59.0.0/19
Challenge: 64.0.0.0/16
Answers
THE OSI MODEL
Application
Presentation
Routing Basics
Session
Transport
Network
Data-Link
Path Determination &
Packet Switching
Physical
Table of Contents
A Router’s Functions
• A router is responsible for determining
the packet’s path and switching the
packet out the correct port.
• A router does this in five steps:
1. De-encapsulates the packet
2. Performs the ANDing operation
3. Looks for entry in routing table
4. Re-encapsulates packet into a frame
5. Switches the packet out the correct
interface
Routed v. Routing Protocols
• What is a routed protocol?
 Routed protocols are protocols that enable data to
be transmitted across a collection of networks or
internetworks using a hierarchical addressing
scheme.
 Examples include IP, IPX and AppleTalk.
 A routable protocol provides both a network and
node number to each device on the network.
Routers AND the address to discover the network
portion of the address.
 An example of a protocol that is not routable is
NetBEUI because it does not have a network/node
structure.
Routed v. Routing Protocols
• What is a routing protocol?
 A routing protocol is a protocol that
determines the path a routed protocol will
follow to its destination.
 Routers use routing protocols to create a
map of the network. These maps allow
path determination and packet switching.
Maps become part of the router’s routing
table.
 Examples of routing protocols include: RIP,
IGRP, EIGRP, & OSPF
Multi-protocol Routing
• Routers are capable of running multiple
routing protocols (RIP, IGRP, OSPF, etc.) as
well as running multiple routed protocols (IP,
IPX, AppleTalk).
• For a router to be able use different routing
and routing protocols, you must enable the
protocols using the appropriate commands.
Dynamic v. Static Routing
• Dynamic routing refers to the process of
allowing the router to determine the path to
the destination.
• Routing protocols enable dynamic routing
where multiple paths to the same destination
exist.
Dynamic v. Static Routing
• Static routing means that the network
administrator directly assigns the path router
are to take to the destination.
• Static routing is most often used with stub
networks where only one path exists to the
destination.
Default Routes
• A default route is usually to a border or
gateway router that all routers on a network
can send packets to if they do not know the
route for a particular network.
Routing Protocol Classes
• Routing protocols can be divided into
three classes:
 Distance–vector: determines the route
based on the direction (vector) and
distance to the destination
 Link-state: opens the shortest path first to
the destination by recreating an exact
topology of the network in its routing table
 Hybrid: combines aspects of both
Convergence
• Convergence means that all routers
share the same information about the
network. In other words, each router
knows its neighbor routers routing table
• Every time there is a topology change,
routing protocols update the routers
until the network is said to have
converged again.
• The time of convergence varies
depending upon the routing protocol
being used.
Distance-vector Routing
• Each router receives a routing table periodically
from its directly connected neighboring routers.
• For example, in the graphic, Router B receives
information from Router A. Router B adds a
distance-vector number (such as a number of
hops), and then passes this new routing table to
its other neighbor, Router C.
Link-state Routing
• Link-state protocols maintain complex databases
that summarize routes to the entire network.
• Each time a new route is added or a route goes
down, each router receives a message and then
recalculates a spanning tree algorithm and
updates its topology database.
Comparing the Two
DISTANCE-VECTOR
LINK-STATE
Views network topology from
neighbor’s perspective
Gets common view of entire
network topology
Adds distance vectors from
router to router
Calculates the shortest path to
other routers
Frequent, periodic updates:
slow convergence
Event triggered updates: fast
convergence
Passes copies of routing tables Passes link-state routing updates
to neighbors
to all routers in the system.
Hybrid Routing
• Cisco’s proprietary routing protocol,
EIGRP, is considered a hybrid.
• EIGRP uses distance-vector metrics.
However, it uses event-triggered
topology changes instead of periodic
passing of routing tables.
THE OSI MODEL
Application
Presentation
Transport Layer
Session
Transport
Network
A Quick Review
Data-Link
Physical
Table of Contents
Transport Layer Functions
• Synchronization of the connection
 Three-way handshake
• Flow Control
 “Slow down, you’re overloading my
memory buffer!!”
• Reliability & Error Recovery
 Windowing: “How much data can I send
before getting an acknowledgement?”
 Retransmission of lost or unacknowledged
segments
Transport’s Two Protocols
• TCP
 Transmission Control
Protocol
 Connection-oriented
 Acknowledgment &
Retransmission of
segments
 Windowing
 Applications:
 Email
 File Transfer
 E-Commerce
• UDP
 User Datagram
Protocol
 Connectionless
 No
Acknowledgements
 Applications:




Routing Protocols
Streaming Audio
Gaming
Video Conferencing