03 04Layers Model

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7 Layer OSI Model


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Seven layers of the OSI model


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Encapsulation

Data exists at each layer contained within a

unit called a Protocol Data Unit (PDU)

The process by which data moves between

PDU types is called Encapsulation

PDU move through interfaces between

layers using Service Access Points (SAP)


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PDUs And the OSI Model

Layer PDU Name

7.) Application Data

6.) Presentation Data5.) Session Data

4.) Transport Segment

3.) Network Packet

2.) Data Link Frame

1.) Physical Bits


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IP Addresses

An IP address is 32-bit binary address that

Uniquely & universally defines connection of ahost or router on Internet

Internet address are usually written in decimalnotation with decimal point separating the bytes


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Classful Addressing - IP Classes

IP address is divided into 5 classes

A, B, C, D, & E


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Finding Class in Binary Notation


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Finding Class in Dotted-Decimal Notation

First byte -> determine class of address

Each class -> specific range of numbers


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Netid and Hostid

Claas A, B & C -> divided into netid & hostid

Class D & E -> not divided into netid & hostid


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Classes and Blocks

Each class is divided into fixed number of blocks with each blockhaving a fixed size

Class A -> divided into 128 blocks Each block have a different netid

1st block -> addresses from 0.0.0.0 to 0.255.255.255 (netid 0)

2nd block -> addresses from 1.0.0.0 to1.255.255.255 (netid 1)Last block -> addresses from 127.0.0.0 to 127.255.255.255 (netid 127)first & last block in each class is reserved for special purposes. Inaddition

One block (netid 10) -> private addresses

total number of organizations that can have class A is only 125Each block in a class contains 16,777,216 address . Millions of class

A addresses are wasted . Next slide shows blocks in class A

graphically


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Blocks in Class A

An organization granted a block with netid 73 uses its addresses First address in block used to identify organization from rest ofInternet -> address is called network address

It defines network of organizations not individual hosts


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Class B

Divided into 16, 384 blocks with each block havingdifferent netid Sixteen blocks reserved for private addresses leaving

16,368 blocks for assignment to organizations

1st block -> addresses from 128.0.0.0 to 128.0.255.255(netid 128.0)

Last block -> addresses from 191.255.0.0 to 191.255.255.255(netid 191.255)

Total number of organizations that can have class Bis 16,368

Each block in a class contains 65,536 classes Next slide shows blocks in class B graphically


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Blocks in Class B

Organization granted block with netid 180.8 First address is network address


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Class C

Divided into 2,097,152 blocks with each blockhaving different netid

256 blocks used for private addresses leaving

2096,896 blocks for assignment to organzations

1st block -> addresses from 192.0.0.0 to128.0.0.255 (netid 192.0.0)

Last block -> addresses from 223.255.255.0 to223.255.255.255 (netid 223.255.255) Next slide

shows blocks in class C graphically


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Blocks in Class C


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Network Address

Network address is one that is assigned toorganization

Range of addresses can automatically be inferred fromnetwork address

Example -> given network address 17.0.0.0, find theclass, the block, and range of addresses

This is class A address -> first byte is between 0 & 127 Block has a netid of 17

Range of addresses range from 17.0.0.0 to 17.255.255.255


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Mask

Mask is 32-bit binary number that gives firstaddress in block i.e., network address when

ANDed bitwise with an address in block


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Default Masks

The network address is the beginning address of each block It can be found

by applying the default mask to any of the addresses in the block (includingitself).

It retains the netid of the block and sets the hostid to zero

Class Mask in binary Mask in-dotted

decimalA 11111111 00000000 255.0.0.0

00000000 00000000

B 11111111 11111111 255. 255.0.0

00000000 00000000

C 11111111 11111111 255. 255. 255.01111111100000000


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Applying the Mask

Given address is 23.56.7.91 & default class Amask, find the network address?

Given address 201.180.56.5 & class Cdefault mask, find network address?


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Example - Network Address


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Multihomed Devices

Multihomed -> A computer connected to different networks

Will have more than one address, each possibly belonging to adifferent class

A router must be connected to more than one networkotherwise -> it cannot route

Router has more than one IP address, one for each interface Figure shows one multihomed computer & one router


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Special IP addresses

IP address 0.0.0.0 is used by hosts when they are booted IP

addresses with 0s as network number refer to the current network

Address with all 1s allows broadcasting on local network All address

of form 127.xx.yy.zz are reserved for loop back testing


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Example - This Host on This Network


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Example - Specific Host on This Network


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Example - Broadcast on Local Network


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Example - Broadcast on Distant Network


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Example - Loopback Address


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Private Addresses

Number of blocks in each class are assigned forprivate use

Not recognized globally

class Netids Blocks

A 10.0.0.0 1

B 172.16 to 172.31 16

C 192.168.0 to 192.168.255 256


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Example - Sample Internet with ClassfulAddresses


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Practice Question 3

Suppose that instead of using 16 bits fornetwork part of a class B address originally,20 bits had been used. How many class B

networks would there have been?


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Solution Practice Question 3

With a 2-bit prefix, there would have been 18bits left over to indicate the network

Consequently, the number of networks would

have been 218 or 262,144. However, all 0s and all 1s are special, so only 262,142

are available.


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Practice Question 4

A network on Internet has a subnet mask of255.255.240.0. What is maximum no. ofhosts it can handle?


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Solution Practice Question 4

The mask is 20 bits long, so the network partis 20 bits

The remaining 12 bits are for the host, so 4096

host addresses exist


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THE END

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03 04Layers Model