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NETWORK LAYER
PHM VN TNH, Ph.D.
PART5OSI NETWORK LAYER
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Objectives
Identify the role of the Network Layer, as it describescommunication from one end device to another enddevice.
Examine the most common Network Layer protocol,Internet Protocol (IP), and its features for providing
connectionless and best-effort service. Understand the principles used to guide the division
or grouping of devices into networks.
Understand the hierarchical addressing of devices andhow this allows communication between networks.
Understand the fundamentals of routes, next hopaddresses and packet forwarding to a destination
network.
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Introduction The protocols of the OSI model Network layer specify
addressingand processes that enable Transport layer data to be
packaged and transported. The Network layer encapsulationallows its contents tobe passed to thedestination within
a network or onanother networkwith minimum
overhead.
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Functions of Network Layer
The major functions of the network layer:
Defines a packet and an addressing scheme.The network layer's addressing scheme is used bydevices to determine the destination of data as itmoves through the network
Find the best path through the network.Path determination is the process that the routeruses to choose the next hop in the path for thepacket to travel to its destination. This process isalso called routing the packet.
Communication between separate networks.
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Role of the Network Layer
Network Layer Communication from Host toHost
Layer 3 uses four basic processes:
Addressing
Encapsulation
Routing: Intermediary devices that connect thenetworks are called routers. The role of the routeris to select paths for and direct packets toward their
destination. This process is known as routing. Decapsulation
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Encapsulation
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De-Encapsulation
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Network Layer Protocols
Protocols implemented at the Network layer that carryuser data include:
Internet Protocol version 4 (IPv4)
Internet Protocol version 6 (IPv6)
Novell Internetwork Packet Exchange (IPX)
AppleTalk
Connectionless Network Service (CLNS/DECNet)
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Network Layer and Internet Protocol
The Internet Protocol was designedas a protocol with lowoverhead.It provides only the functions that are necessary to
deliver a packet from a source to a destination over aninterconnected system of networks
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The IPv4 Protocol Connectionless
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The IPv4 Protocol Best Effort
Best Effort Service (unreliable)
Describe the implications for the use of the IP protocolas it is considered an unreliable protocol
Unreliable means simply that IP does not havethecapability to manage, and recover from, undelivered
or corrupt packets
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The IPv4 Protocol Media Independent
IPv4 and IPv6 operate independently of the media thatcarry the data at lower layers of the protocol stack
One major characteristic of the media that the Networklayer considers: the maximumsize of PDU that eachmedium can transport:the MaximumTransmissionUnit (MTU)
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Packaging the Transport Layer PDU
The process of encapsulating data by layer enablesthe services at the different layers to develop andscale without affecting other layers.
Routers can implement these different Network layerprotocols to operate concurrently over a network to
and from the same or different hosts.
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IPv4 Packet Header
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Network layer fields
indicates the version of
IP currently used (4 bits)
indicates the datagram
header length in 32 bit
words (4 bits)
specifies the level of
importance that a particular
upper layer protocol hasassigned (8 bits)
specifies the length of the entire
IP packet, including data and
header, in bytes (16 bits)contains an integer that identifies
the current datagram (16 bits) This
is the sequence number
3-bit field in which the 2 low-order bits control
fragmentation - one bit specifying whether the
packet can be fragmented, and the second
whether the packet is the last fragment in a
series of fragmented packets (3 bits)
the field that is used to help
piece together datagram
fragments (13 bits)
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Network layer fields
maintains a counter that gradually
decreases, by increments, to zero, at
which point the datagram is
discarded, keeping the packets from
looping endlessly (8 bits)
indicates which upper-layer protocol,such as TCP or UDP, receives
incoming packets after IP processing
has been completed (8 bits)
helps ensure IP header
integrity (16 bits)
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Network layer fields
specifies the sending node IP
address, 32 bitsspecifies the receiving node IP
address, 32 bits
allows IP to support various
options, such as security,
variable length.
extra zeros are added to this field
to ensure that the IP header is
always a multiple of 32 bits
contains upper-layer information,
variable length up to 64 KB
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Networks
Dividing Hosts into Groups
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Separating Hosts into Common Groups
Networks can be grouped based on factors thatinclude:
Geographic location
Purpose
Ownership
Geographic
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Separating Hosts into Common Groups
Purpose: Users who have similar tasks typically usecommon software, common tools, and have commontraffic patterns
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Separating Hosts into Common Groups
Ownership: Using an organizational (company,department) basis for creating networks assists incontrolling access to the devices and data as well asthe administration of the networks
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Separating Hosts into Common Groups
Ownership
h h i k
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Why separate hosts into networks
Common issues with large networks are: Performancedegradation, Security issues, Address Management
h h i k
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Why separate hosts into networks
Increase network security
Wh h i k
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Why separate hosts into networks
Address management: To expect each host toknow the address of every other host would impose aprocessing burden on these network devices thatwould severely degrade their performance.
Wh t h t i t t k
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Why separate hosts into networks
Hierarchical addressing: solves the problem of devicescommunicating across networks of networks
i idi h k k f k
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Dividing the networks -Networks from networks
If a large network has to be divided, additional layersof addressing can be created. Using hierarchicaladdressing means that the higher levels of theaddress are retained; with a subnetwork level andthen the host level
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Routing
Supporting communication outside our network
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Supporting communication outside our network
To communicate with a device on another network, ahost uses the address of this gateway, or defaultgateway, to forward a packet outside the localnetwork
The router also needs a route that defines where to
forward the packet next. This is called the next-hopaddress. If a route is available to the router, the routerwill forward the packet to the next-hop router thatoffers a path to the destination network.
P k t F di
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Packet Forwarding
Describe the role of an intermediary gateway device inallowing devices to communicate across sub-dividednetworks
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If the destination host is in the same network as the sourcehost, the packet is delivered between the two hosts on the local
media without the need for a router. If the destination host and source host are not in the same
network, the packet may be carrying a Transport layer PDUacross many networks and through many routers.
IP Packet Ca ing Data End to End
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IP Packet Carrying Data End-to-End
IP Packet Carrying Data End to End
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IP Packet Carrying Data End-to-End
IP Packet Carrying Data End to End
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IP Packet Carrying Data End-to-End
IP Packet Carrying Data End to End
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IP Packet Carrying Data End-to-End
IP Packet Carrying Data End to End
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IP Packet Carrying Data End-to-End
A gateway the way out of our network
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A gateway the way out of our network
A gateway the way out of our network
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A gateway the way out of our network
A gateway the way out of our network
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A gateway the way out of our network
A router makes a forwarding decision for each packet thatarrives at the gateway interface. This forwarding process is
referred to as routing. To forward a packet to a destinationnetwork, the router requires a route to that network. If a routeto a destination network does not exist, the packet cannot beforwarded
Routing table
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Routing table
The routing table stores information about connectedand remote networks. Routes in a routing table have
three main features:
Destination network
Next-hop
Metric
Packet forwarding
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Packet forwarding
Routing is done packet-by-packet and hop-by-hop. Each packetis treated independently in each router along the path.
The router will do one of three things with the packet: Forwardit to the next-hop router; Forward it to the destination host;Drop it
Packet forwarding
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Packet forwarding
If the routing table does not contain a more specific route entryfor an arriving packet, the packet is forwarded to the interface
indicated by a default route, if one exists. The default route isalso known as the Gateway of Last Resort.
Packet forwarding
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Packet forwarding
Routing Protocol- Sharing the route
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Routing Protocol- Sharing the route
Provides processes for sharing route information
Allows routers to communicate with other routers to update
and maintain the routing tables Examples: Routing Information Protocol (RIP), Interior Gateway
Routing Protocol (IGRP), Open Shortest Path First (OSPF),Border Gateway Protocol (BGP), and Enhanced IGRP (EIGRP)
Routing protocol Sharing the route
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Routing protocol Sharing the route
Routing protocols: static and dynamic routes
Static Routing
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Static Routing
Static route: routes to remote networks with theassociated next hops can be manually configuredon
the router. A default route can also be staticallyconfigured
Dynamic Routing
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Dynamic Routing
Routing protocols are the set of rules by which routersdynamically share their routing information
IP Addressing
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IP Addressing
Addressing is a key function of Network layerprotocols that enables data communication between
hosts on the same network or on different networks
IP addressing
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IP addressing
For any two systems to communicate, they
must be able to identify and locate each other
IP addressing
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IP addressing
AmericanRegistry forInternetNumbers
(Ti Vit Nam :VNNIC)
IP Addressing Structure
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IP Addressing Structure
IP Address converting
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IP Address converting
Classify and Define IPv4 Addresses
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Classify and Define IPv4 Addresses
1
Class A:
Bits:
0NNNNNNN0NNNNNNN HostHost HostHost HostHost
8 9 16 17 24 25 32
Range (1-126)
1
Class B:
Bits:
10NNNNNN10NNNNNN NetworkNetwork HostHost HostHost
8 9 16 17 24 25 32
Range (128-191)1
Class C:
Bits:
110NNNNN110NNNNN NetworkNetwork NetworkNetwork HostHost
8 9 16 17 24 25 32
Range (192-223)
1
Class D:
Bits:
1110MMMM1110MMMM Multicast GroupMulticast Group Multicast GroupMulticast Group Multicast GroupMulticast Group
8 9 16 17 2425 32
Range (224-239)
1
Class A:
Bits:
0NNNNNNN0NNNNNNN HostHost HostHost HostHost
8 9 16 17 24 25 32
Range (1-126)
1
Class B:
Bits:
10NNNNNN10NNNNNN NetworkNetwork HostHost HostHost
8 9 16 17 24 25 32
Range (128-191)1
Class C:
Bits:
110NNNNN110NNNNN NetworkNetwork NetworkNetwork HostHost
8 9 16 17 24 25 32
Range (192-223)
1
Class D:
Bits:
1110MMMM1110MMMM Multicast GroupMulticast Group Multicast GroupMulticast Group Multicast GroupMulticast Group
8 9 16 17 2425 32
Range (224-239)
Classify and Define IPv4 Addresses
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Classify and Define IPv4 Addresses
IP address classes ranges
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IP address classes ranges
CL
A
SS
1stOctetDec.
Range
1st OctetHigh
Order Bits
Network /Host ID
(N=Network,
H=Host)
Default SubnetMask
Number ofNetworks
Hosts perNetwork (usable
addresses)
A 1126*
0 N.H.H.H 255.0.0.0 126(272)
16,777,214(2 242)
B 128
191
1 0 N.N.H.H 255.255.0.0 16,382
(214- 2)
65,534
(2 162)
C 192223
1 1 0 N.N.N.H 255.255.255.0 2,097,150(2212)
254(2 82)
D 224239
1 1 1 0 Reserved for Multicasting
E 240254
1 1 1 1 0 Experimental, used for research
* Class A address 127 cannot be used and is reserved for loopback
and diagnostic functions
IP address classes ranges - summary
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IP address classes ranges summary
Type of Address in an IPv4 Network
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Type of Address in an IPv4 Network
Three types of addresses:
Network address -The address by which we refer tothe network. All hosts in a network will have the samenetwork bits.
Broadcast address-A special address used to send
data to all hosts in the network. The broadcastaddress uses the highest address in the networkrange. This is the address in which the bits in the hostportionare all 1s. This address is also referred to as
the directed broadcast. Host addresses -The addresses assigned to the end
devices in the network
Type of Address in an IPv4 Network
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Type of Address in an IPv4 Network
Type of Address in an IPv4 Network
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yp
Type of Address in an IPv4 Network
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yp
Type of Address in an IPv4 Network
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yp
Network Address : All Host Bits = 0
Broadcast Address :All Host Bits = 1
Class A : 11.0.0.0
Class B : 172.16.0.0
Class C : 201.201.201.0
Class A : 11.255.255.255Class B : 172.16.255.255
Class C : 201.201.201.255
Subnet Mask
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Defining the Network and Host Portions
The subnet mask is 32-bit patternand created by placing a
binary 1 in each bit position that represents the networkportionand placing a binary 0in each bit position thatrepresents the host portion.
The prefix and the subnet mask are different ways of
representing the same thing -the network portion of anaddress
In 8-bit pattern, there are:
00000000 = 0 11110000 = 240
10000000 = 128 11111000 = 24811000000 = 192 11111100 = 25211100000 = 224 11111110 = 254
11111111 = 255
Defining the Network and Host Portions
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g
Defining the Network and Host Portions
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g
Defining the Network and Host Portions
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g
ANDing process: extracts the network address fromthe IP address.
Defining the Network and Host Portions
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g
Type of Communication
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yp
Unicast: Is used for the normal
host-to-hostcommunication in both a
client/server and a peer-to-peer network.
Uses the host address ofthe destination device as
the destination address andcan be routed through aninternetwork.
Three types: Unicast, Broadcast, Multicast
Type of Communication
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Broadcast:
The process of sending a packet
from one host to all hosts in thenetwork
Host processes a broadcastaddress destination packet like
unicast address. A directed broadcast is sent
to all hosts on a specificnetwork.
The limited broadcastis used for communicationthat is limited to the hosts onthe local network.
Type of Communication
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Multicast:
The process of sending a packetfrom one host to a selectedgroup of hosts.
Multicast transmission is
designed to conserve thebandwidth of the IPv4 network.
The multicast clients useservices initiated by
a client program tosubscribe to themulticast group.
Public and Private addresses
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Public and Private addresses
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Private Addresses:are set aside for use in privatenetworks.
10.0.0.0 to 10.255.255.255 (10.0.0.0 /8)
172.16.0.0 to 172.31.255.255 (172.16.0.0 /12)
192.168.0.0 to 192.168.255.255 (192.168.0.0 /16)
Public Addresses:are designed to be used in thehosts that are publicly accessible from the Internet.
Network Address Translation (NAT):is used to
translate private addresses to public addresses, beimplemented on a device at the edge of the privatenetwork.
Special IPv4 Addresses
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Network Addresses
Broadcast Addresses
Default Route0.0.0.0.
Link-Local Addresses
169.254.0.0 to 169.254.255.255 (169.254.0.0 /16)
These addresses can be automatically assigned
TEST-NET Addresses
The address block 192.0.2.0 to 192.0.2.255 (192.0.2.0
/24) is set aside for teaching and learning purposes. Theseaddresses can be used in documentation and networkexamples. Unlike the experimental addresses, networkdevices will accept these addresses in their configurations
Special IPv4 Addresses
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Assigning Addresses
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Static Assignment: The network administrator mustmanually configure the network information for a host
Assigning Addresses
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Dynamic Host Configuration Protocol (DHCP):enables the automatic assignment of addressing
information such as IP address, subnet mask, defaultgateway, and other configuration information.
Overview of IPv6
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In the early 1990s, the Internet Engineering TaskForce (IETF) grew concerned about the exhaustion of
the IPv4 network addresses and began to look for areplacement for this protocol. This activity led to thedevelopment of what is now known as IPv6.
Overview of IPv6
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Creating expanded addressing capabilities was the initialmotivation for developing this new protocol. Other issues were
also considered during the development of IPv6, such as:Improved packet handling
Increased scalability and longevity
QoS mechanisms
Integrated security To provide these features, IPv6 offers:
128-bit hierarchical addressing -to expand addressingcapabilities
Header format simplification -to improve packet handling
Improved support for extensions and options -for increasedscalability/longevity and improved packet handling
Flow labeling capability -as QoS mechanisms
Authentication and privacy capabilities -to integrate security
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Calculating IP Addresses
Introduction to subnetting
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Basic subnetting
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Basic subnetting
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Basic subnetting
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Dividing Networks into Right Sizes
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Extract network addresses from host addresses usingthe subnet mask. Total host 800 -> choose block
172.16.0.0/22
SUBNET SCHEME
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IP Class C : 192.168.10.0 Borrowed bits : 3Max. IP Number : 32Max. Subnet Number : 8
Subnet Mask : 255.255.255.224
Subnet 0: Subnetwork ID = Major Network Number
Subnet 7: Subnetwork Broadcast ID = Major Network Broadcast
192.168.10.0 : 192.168.10.255
Subnetting a Subnet
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Subnetting a Subnet
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Network Layer Protocols
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IP provides connectionless, best-effort deliveryrouting of packets. IP is not concerned with the
content of the packets but looks for a path to thedestination.
Internet Control Message Protocol (ICMP) providescontrol and messaging capabilities.
Address Resolution Protocol (ARP) determines thedata link layer address, MAC address, for known IPaddresses.
Reverse Address Resolution Protocol (RARP)determines IP addresses when the MAC address isknown
ICMPv4
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The purpose of these messages is to provide feedbackabout issues related to the processing of IP packets
under certain conditions, not to make IP reliable. ICMPmessages are not required and are often not allowedfor security reasons.
ICMP is the messaging protocol for the TCP/IP suite.ICMP provides control and error messages and is usedby the ping and traceroute utilities. Although ICMPuses the basic support of IP as if it were a higher-level
protocol ICMP, it is actually a separate Layer 3 of theTCP/IP suite.
ICMPv4
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The types of ICMP messages -and the reasons whythey are sent -are extensive. We will discuss some of
the more common messages. ICMP messages thatmay be sent include:
Host confirmation
Unreachable Destination or Service
Time exceeded
Route redirection
Source quench
ICMP - Testing the Local Stack
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Pinging the Local Loopback: 127.0.0.1
ICMP-Testing Connectivity to the Local LAN
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Ping Gateway
ICMP - Testing the Path
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Use tracert/traceroute to observe the path betweentwo devices as they communicate and trace the steps
of tracert/traceroute's operation
IP address assignment
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staticaddressing and dynamicaddressing
Static addressing
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You must go to each individual device and
configure it with an IP address. You should keep very meticulous records,
because problems can occur on the network ifyou use duplicate IP addresses.
Dynamic addressing
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There are a few different methods that you canuse to assign IP addresses dynamically:
RARP:Reverse Address Resolution Protocol.
BOOTP:BOOTstrap Protocol.
DHCP:Dynamic Host Configuration Protocol.
Dynamic addressing:RARP
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MAC: Known
IP: Unknown
RARP Request
RARP Reply
RARP server
Dynamic addressing:BOOTP
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MAC: Known
IP: Unknown
UDP Broadcast
UDP Broadcast
BOOTP server
MAC1IP1MAC2IP2MAC3IP3
IP Address
GatewayIP of serverVendor-specific
Dynamic addressing:DHCP
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MAC: Known
IP: Unknown
DHCP Discover
UDP Broadcast
DHCP Offer
UDP Unicast
DHCP server
IP1IP2IP3
DHCP Request
UDP Broadcast
DHCP Ack
IP AddressGatewayIP of serversAnd more
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ARP CONCEPT
Encapsulation
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Address resolution protocol
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In order for devices to communicate, the sendingdevices need boththe IP addresses and the MACaddressesof the destination devices.
When they try to communicate with devices whoseIP addresses they know, they must determine theMAC addresses.
ARPenables a computer to findthe MACaddressof the computer that is associated with an IPaddress.
Address resolution protocol
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ARP table in host
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ARP operation
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10.0.2.1
A.B.C.1.2.3
10.0.2.9
A.B.C.7.8.9
10.0.2.5
A.B.C.4.5.6
A B C
ARP Table:
? MACA.B.C.1.2.3
MAC?
IP10.0.2.1
IP10.0.2.9
Data
ARP operation:ARP request
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10.0.2.1
A.B.C.1.2.3
10.0.2.9
A.B.C.7.8.9
10.0.2.5
A.B.C.4.5.6
A B C
MACA.B.C.1.2.3
MACff.ff.ff.ff.ff.ff
IP10.0.2.1
IP10.0.2.9
What is your MAC Addr?
ARP operation:Checking
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10.0.2.1
A.B.C.1.2.3
10.0.2.9
A.B.C.7.8.9
10.0.2.5
A.B.C.4.5.6
A B C
MACA.B.C.1.2.3
MACff.ff.ff.ff.ff.ff
IP10.0.2.1
IP10.0.2.9
What is your MAC Addr?
ARP operation:ARP reply
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10.0.2.1
A.B.C.1.2.3
10.0.2.9
A.B.C.7.8.9
10.0.2.5
A.B.C.4.5.6
A B C
MACA.B.C.7.8.9
MACA.B.C.1.2.3
IP10.0.2.9
IP10.0.2.1
This is my MAC Addr
ARP operation:Caching
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10.0.2.1
A.B.C.1.2.3
10.0.2.9
A.B.C.7.8.9
10.0.2.5
A.B.C.4.5.6
A B C
ARP Table:
A.B.C.7.8.910.0.2.9
MACA.B.C.1.2.3
MACA.B.C.7.8.9
IP10.0.2.1
IP10.0.2.9
Data
ARP:Destination local
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Internetwork communication
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How to communicate with devices that are not onthe same physical network segment.
Default gateway
I d f d i i i h h
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In order for a device to communicate with anotherdevice on another network, you must supply it with
a default gateway. A default gateway is the IP address of the interface
on the router that connects to the networksegment on which the source host is located.
In order for a device to send data to the address ofa device that is on another network segment, thesource device sends the data to a default gateway.