Download IP address

Chapter 4
Network
Layer
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OBJECTIVES
After reading this chapter, the reader should
be able to:
Understand the position of the network layer in the Internet
model.
Understand the rationale for the existence of the network layer.
Understand the concept of host-to-host delivery.
Understand the duties of the network layer: packetizing,
addressing, and routing.
Understand the network layer protocol, IP, used in the Internet.
Know which upper-layer protocol can use the services of IP.
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4.1
INTRODUCTION
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Figure 4-1
Network layer in the Internet model
 The network layer provides host-to-host delivery.
 Host-to-host : from the source computer to the destination computer.
 A network-layer protocol takes a message from a computer and
delivers it to another computer.
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Figure 4-2
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Duties of the network layer
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4.2
ADDRESSING
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Note:
The network addresses must be
unique.
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At the network layer, we need to uniquely identify each device on
the Internet to allow global communication between all devices.
(analogous to the telephone system)
The addresses used in the network layer must uniquely and universally
define the connection of a host (computer) or a router to the Internet.
IP Address
Addresses used in the Internet are called IP address.
An IP address is made of 4 bytes(32 bits), where each byte can
have a value between 0 and 255.
The bytes are separated by a dot(.) when used by people and
programs.
Ex: 10.34.234.8
The notation is referred to as dotted-decimal notation.
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Technical Focus:
Binary Notation of IP Addresses
An IP address is stored as a binary number in the computer. A
4-part dotted-decimal address can be converted to binary if we
replace each part by its binary equivalent. The following
shows the IP address 10.34.234.8 in dotted-decimal and binary
notation:
10.34.224.8
00001010 00100010 11100000 00001000
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Technical Focus:
Address Space
A protocol such as IP that defines addresses has an address
space. An address space is the total number of addresses
available to the protocol. If a protocol uses N bits to define an
address, the address space is 2N because each bit can have two
different values (0 and 1); N bits can have 2N values.
The Internet uses 32-bit addresses, which means that the
address space is 232 or 4,294,967,296 (more than 4 billion).
This means that theoretically, if there were no restrictions,
more than 4 billion devices could be connected to the Internet.
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Two-Level Address: Network and Host
 If an organization has only one single network( which is improbable ),two
levels of addressing is adequate.
 Ex:
 Figure 4.3 shows a hypothetical network in an organization.
 The organization is granted 65536 addresses from the Internet
authorities.
 The addresses range from 181.74.0.0 to 171.74.255.255.
 Note:
 This range contains 65536 addresses because the third and the fourth bytes
can have values of 0 to 255 and 256*256 is 65536.
 The first address,181.74.0.0 is a reserved address and represents the entire
network( organization); the last address 181.74.255.255 is also a reserved
address and represents a limited broadcast address( used in case a host
wants to send a message to every host.)
 The remaining addresses belong to the organization and can be assigned to
hosts.
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Figure 4-3
Two levels of hierarchy
All hosts always have the first two parts of the address(181.74)in common.
The common part of the address is called the netid (or prefix); the value part is called
the hostid.
The first address in the group,which always has a hostid of zero,is called the
network address.
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Note:
When using two levels of address
hierarchy, the common part is
referred to as the netid or prefix
and the variable part is referred
to as the hostid, or suffix.
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Three-Level Address: Network,
Subnetwork and Host
• In this three-level addressing scheme,the individual LANs are referred to as
subnetworks(or subnets) while the whole system is referred to as a network
(or site).
• The addresses can reflect this hierarchy in the physical network.
• Ex:
– If the campus is granted 65536 addresses,the first 2 bytes can define the whole
network (site); the third byte can define the subnet; and the fourth byte can
define a host on a subnet.
– The first 2bytes,are the same for all computers.
– The third byte is the same for those computers connected to the same
subnet,and the fourth byte is unique for each computer.
• Some addresses have special meaning.
– The first address defines the entire site(campus).
– The first address of each subnet define the subnet address.
– The last address of each subnet is used for broadcasting to all computers in the
same subnet.
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Figure 4-4
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Three levels of hierarchy
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Note:
With three levels of address
hierarchy, the common part is
referred to as the netid. The part
common to all computers
connected to the same subnet is
referred to as the subnetid. The
unique part is referred to as the
hostid.
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Note:
A packet traveling from the source to the
destination needs at least four addresses: source
and destination port numbers and source and
destination IP addresses.
Application-layer address www.fhda.edu
Transport-layer address46017,80(client and server port number)
Network-layer address201.23.56.8,181.17.43.56(source and
destination IP addresses)
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• Domain Name System
– Application-Layer protocol
– A computer that needs to obtain an IP destination address
uses the client DNS.
– A computer that stores the mapping between the
application-layer address and the IP address used the server
DNS.
– Every computer knows the address of at least one DNS
server.
– It sends its request to that server.
– Either the server knows the IP address corresponding to the
application-layer address, or it knows another server that
does.
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Figure 4-5
DNS example
A user is sitting at a computer with IP address 200.31.45.8 and wants
to send a message to the company with web(application-layer)address
www.goodproduct.com.
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4.3
ROUTING
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Routing
• Network cables are plugged into routers or hubs,
which help direct information transfer across the
network.
• Ex: figure 4.6
– A packet starting from host A can follow two different
routes to reach host B.
• It can go through LAN1,R1(a router),LAN3,R2,WAN1,R5, and
LAN6.
• Or go through LAN1,R1,LAN3,R2,LAN5,R4,WAN2,R6,and
LAN6.
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Figure 4-6
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Example of portion of the Internet
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• There are two questions here:
– Which of the above routes must the packet take?
– When a choice is made,how is the packet routed to reach
the final destination?
• The answer to the first question depends on the type
of service (TOS) required by the packet.
– Ex: figure4.7
• Host A can include a field in the packet that specifies
that the packet is to travel through a minimum delay
route or a reliable route. (The routers can use the value
in this field to make a decision.)
• Videoconferencing requires minimum delay.
• File transfer requires reliability.
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Figure 4-7
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Routing decisions
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Figure 4-8
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Routing table for router R1
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Figure 4-9
Routing tables
The answer to the second question is routing tables.
Each router must have (at least) two tables:
One table to route packets that need minimum delay and
another for packets that need reliability.
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Note:
Routing tables can be based on
next-hop routing.
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Size of routing table
• Host-specific: each row is based on the host
address.
• Network specific: each row is based on the
network address.
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Note:
Routing tables for the Internet are
usually network-specific rather than
host-specific. The router routes the
packet to the final network; it is then
broadcast to reach the final
destination.
Network-based routing reduces the
size of routing tables.
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Autonomous systems
• Another way to reduce the size of a routing table is to
divide the Internet into smaller sections called
autonomous systems.
• Autonomous systems(AS) is a group of networks and
routers under the authority of a single administration.
• Routing inside an autonomous systems is referred to
as interior routing.
• Routing between autonomous systems is referred to
as exterior routing.
• This is also called hierarchical routing.
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Figure 4-10
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Autonomous systems
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Note:
Routing in the Internet is
hierarchical. Delivery is first
made to the autonomous system,
then to the network, and finally to
the host. Hierarchical routing
reduces the size of the routing
tables.
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• Static Versus Dynamic Routing
– Static Routing Table
– Dynamic Routing Table
• Routing Protocols
– RIP(Routing Information Protocol)
• An interior routing protocol used inside an autonomous system.
• In distance vector routing ,each router periodically shares its
knowledge about the entire internet with its neighbors.
• The three keys to understanding how this algorithm works are as
follows:
– Sharing knowledge about the entire autonomous system.
– Sharing only with neighbors.
– Sharing at regular interval.
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– OSPF(Open Shortest Path First)
• Another interior routing protocol that is gaining in
popularity.
• Its domain is also autonomous system.
• Link state routing is a process in which each router
shares its knowledge about its neighborhood with every
router in the area.
• The three keys to understanding how this algorithm
works are as follows:
– Sharing knowledge about the neighborhood.
– Sharing with every other router.
– Sharing when there is a change.
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– BGP(Border Gateway Protocol)
• An inter-autonomous system routing protocol.
• BGP is based on a routing method called path vector routing.
• Path vector routing is different from both distance vector routing
and link state routing.
• Each entry in the routing table contains the destination network, the
next router, and the path to reach the destination.
Table 4.1 Path vector routing table
Network
N01
N02
N03
N04
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Next Router
R01
R05
R06
R12
Path
AS14,AS23,AS67
AS22,AS67,AS05,AS89
AS67,AS89,AS09,AS34
AS62,AS02,AS09
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4.4
PACKETIZING:
IP PROTOCOL
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Figure 4-11
Position of IP in Internet model
The Internet Protocol(IP) is the transmission mechanism used by the TCP/IP protocols.
IP is an unreliable and connectionless datagram protocol-a best-effort delivery service.
The term bet-effort means that IP provides no error checking or tracking.
If reliability is important ,IP must be paired with a reliable protocol such as TCP.
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Figure 4-12
IP datagram format
Packets in the IP layer are called datagrams.
A datagram is a variable-length packet consisting of two parts:
header and data.
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Technical Focus:
Fields in a Datagram
The fields in a datagram are as follows:
Version
Protocol
Header length
Checksum
Differentiated service
Source address
Total length
Destination address
Identification, flags, and fragmentation offset
Time to live
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4.5
FRAGMENTATION
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• MTU: Maximum Transfer Unit
• Fragmentation is done by the source host or
routers in the path.
• Reassembly of datagram is done by the
destination host.
• Defining a fragment
– Datagram id
– Last datagram flag
– offset
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