12 - IP Subnetting

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12 - IP Subnetting

By Muhammad Asghar Khan

Reference: CCENT/CCNA ICND1 Official Exam Certification Guide By Wendell Odom


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Agenda

www.asghars.blogspot.com2

Intro to Binary Numbers Subnet Mask

Subnetting

Subnetting Based on Networks Subnetting Based on Hosts

IP Address Reverse Engineering

Rule of Exception Resident Subnet of an IP / How Router Find

the Subnet No

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Intro to Binary Numbers


They only contain 0s and 1s The binary system is also called a 2-based

numbering system, or simply base 2

The binary number 10101 can be converted todecimal number as:

1*24 + 0*23 + 1*22 + 0*2 1+ 1*20 = 21

Or

1*16 + 0*8 + 1*4 + 0*2 + 1*1 = 21

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Intro to Binary Numbers


The decimal number 21 can be converted to binaryas:

LSB

MSB

(21)10 = 10101

2 21 - 1

10 - 0

5 - 1

2 - 0

1

2

2

2LSB

MSB

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Subnet Mask


Subnet mask is a 32-bit value that allows the IP host to learn the

size of network and host part of an IP address

In binary form subnet mask have all 1s in the network and sub-network portion (on the left) and all 0s in the host portion (onthe right)

Every class has its default subnet mask as:

Class A: 255 . 0 . 0. 0

11111111 . 00000000 .00000000 .00000000

Class B: 255 . 255 . 0 . 0

11111111 . 11111111 . 00000000 . 00000000

Class C: 255 . 255 . 255 . 0

11111111 . 11111111 . 11111111 . 00000000

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Network Part(8-

bits)

Host Part (24-bits)

Network Part(16-bits) Host Part(16-bits)

Network Part (24-bits) Host Part(8-bits)


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Subnet Mask


It can be observed that masks are typically written in dotteddecimal number

We can also write or represent the masks in prefix or CIDR

(Classless Inter Domain Routing) notation

In CIDR or prefix notation the binary 1s in subnet mask ispreceded by a /, for example:

Decimal Notation: 255.255.255.0

Binary Notation: 11111111.11111111.11111111.00000000CIDR Notation: /24

The following IP address is mentioned with its mask in CIDR

notation: 192.0.0.0/24

Network Part (24-bits)

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Subnetting


A classful network (A, B & C) is simply a set of consecutivelynumbered IP addresses

Subnetting subdivides these classful network into smaller,logical groups of addresses called subnets

Subnet is shorthand for sub divided network

With no subnets, the network has a flat topolgy

A flat topology relies on L2 MAC addresses to deliverpackets

In flat topology as the network grows, the use of thenetwork bandwidth becomes less efficient, routers can beused in this case to separate networks by breaking thenetwork into multiple subnets

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Subnetting


As we learn that each IP address in class (A, B & C) has adefault subnet mask associated with it

These masks define the pre-defined number of network and

host parts

In subnetting the default subnet mask is customizedaccording to our needs i.e. the host part will be shrink to

make room for the subnet field(s), but the network part will

remain the same

Subnetting can be based on the following scenarios:

Subnetting Based on Networks

Subnetting Based on Hosts

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Subnetting


Subnetting Based on Networks

The following steps can be used to find the subnets basedon number of networks

1. Determine number of networks & convert to binary

2. Reserve bits in subnet i.e customize the default subnet &find the increment

3. Use the increment to find network ranges

Example; Divide the 216.21.5.0 (Class C) network into 5subnets

1. Determine number of networks & convert to binary128 64 32 16 8 4 2 1

1 0 1

=> 3-bits

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Subnetting


2. Reserve bits in subnet & find the increment

Default Mask: 255.2555.255.0

11111111.11111111.11111111.00000000

Customized Mask: 1111111.11111111.11111111.11100000

255.255.255.224

Increment will be the lowest network bit value converted to

decimal

1111111.11111111.11111111.11100000=> 32

3. Use increment to find network ranges

216.21.5.0 --- 216.21.5.31

3-bits reserved for

subnets

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Subnetting


216.21.5.32 --- 216.21.5.63

216.21.5.64 --- 216.21.5.95

216.21.5.96 --- 216.21.5.127

216.21.5.128 --- 216.21.5.160

Note; Every subnet has two reserved IP addresses

called Network ID (Zero Subnet) i.e the first address of

subnet and the last one which is called the broadcast

address (Broadcast Subnet)

To find the total possible of subnets with 3-bits of

subnet field we can use the following formula:

Total subnets = 2s (where s is the subnet bits)

= 23 = 6

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Subnetting


Subnetting Based on Hosts

The same previous steps can be used to find the subnetsbased on number of networks

1. Determine number of hosts & convert to binary

2. Reserve bits in subnet i.e customize the default subnet &find the increment

3. Use the increment to find network ranges

Example; Divide the network 10.0.0.0 (Class A) network intosubnets with 100 hosts each

1. Determine number of hosts & convert to binary128 64 32 16 8 4 2 1

1

=> 7-bits (Bcz if we take 128 then 128 > 100)

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Subnetting


2. Reserve bits in subnet & find the increment

Default Mask: 255.0.0.0 => /8

11111111.00000000.00000000.00000000

Note: Here we are concerned with no. of hosts, so we will reserve

the host bits

Customized Mask:1111111.11111111.11111111.10000000

255.255.255.128 => /25

Again, increment will be the lowest network bit value

converted to decimals

1111111.11111111.11111111.10000000

=> 128

7-bits reserved for

hosts

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Subnetting


3. Use increment to find network ranges

10.0.0.0 --- 10.0.0.127

10.0.0.128 --- 10.0.0.255

10.0.1.0 --- 10.0.1.127

- - - - - - - - - - - - - - - - - Total subnets = 2s = 217 = 131072

To find the total possible of hosts/subnet with 7-bits of

host field we can use the following formula:

Total No. of hosts/subnet = 2h

- 2 (where h is the hosts bits)= 27 2 = 128

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The purpose of reverse engineering is to check thevalidity of the IP address, i.e.

Is the address on the same network as that of other

hosts plug onto the same switch, or

What network range it belongs

Example; check the IP address 192.168.11.32/28 is

valid or not

1. Determine number of subnets bitsDefault Mask: 255.255.255.0 => /24

=> 28 24 = 4 bits

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2. Determine the increment11111111.11111111.11111111.11110000

128 64 32 16 8 4 2 1

=> Lowest Bit = 16

3. Find network ranges

192.168.11.0 --- 192.168.11.15

192.168.11.16 --- 192.168.11.31192.168.11.32 --- 192.168.11.47

192.168.11.48 --- 192.168.11.63

- - - - - - - - - - - - - - - - - - - - - - - -

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The given IP address falls in the range 192.168.11.32to 192.168.11.47, as it is the network or subnet ID,

therefore, it is not a valid IP address

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Rule of Exception


The subnetting process works by finding thenumber of bits that are required to meet a givenrequirement

For example; to find a range of 25 networks, you

need a 5 subnet bits to satisfy the requirement,however, because binary numbers start from 0,there are exceptions to this rule i.e:

For network ranges 2, 4, 8, 16, 32, 64 and 128 always

subtract 1 when finding the networks For number of hosts 3, 7, 15, 31, 63, 127 always add 1

when finding the number of hosts

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Rule of Exception


Thus to be on the safer side, to save bits alwayssubtract 1 from number of subnets and always add

1 to the number of hosts

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Resident Subnet of an IP / How Router Find the

Subnet No


The router use the boolean AND operation to findthe subnet ID or resident subnet of the given IP

address Convert IP & Mask tobinary

1

Perform the Boolean AND on IP and

Mask2

Convert the resulting binary number, 8 bits at a

time , back to decimal. This value is the subnet

number 3

Note:

1111111 (255) when ANDed with IP octet result in the same octet value (as with the

case

in first two octets in our example).

Similarly, any IP address octet, ANDed with 00000000 (0) yields an octet of eight

binary 0s .

So you can guess its value without converting to binary.

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12 - IP Subnetting