Fast and Easy Subnetting

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24 Οκτ 2013 (πριν από 4 χρόνια και 15 μέρες)

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Fast and Easy Subnetting

Marc Khayat, Technical Advocacy Team

makhayat@cisco.com

May 2011

© 2011 Cisco and/or its affiliates. All rights reserved.

2


Number of required subnets


Number of required hosts per subnet

© 2011 Cisco and/or its affiliates. All rights reserved.

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Using the numerical calculations


Using the graphical representation


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Network size is the number of IP addresses in a subnet, including the
network ID and the broadcast address


Magic
nb

is the same as the network size if prefix > /24


Magic
nb

should always be between 1 and 255. If greater than 255,
divide by 256. If smaller than 1, multiply by 256.


Every time you divide by 256, you move 1 octet to the left


Every time you multiply by 256, you move 1 octet to the right


Everything is a multiple of the magic
nb

(therefore, the name

)


Don’t bother
googling

the
term
“magic
nb




Numerical Calculations

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Nb of required subnets should be an exponential multiple of 2 (so, 2, 4,
8, 16, 32, etc.). If not, then round up.


To get the magic
nb
, few easy steps:

1.
Identify the first non
-
255 number in the mask

2.
Subtract that number from 256

3.
Divide the outcome by the number of required subnets

4.
That’s your magic
nb
!

Numerical Calculations

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Required number of subnets 5 (should be really looking at 8)


Mask: (/24) 255.255.255.0

First non
-
255 number is 0

256


0 = 256

256 / 8 = 32 (that’s the magic number)


Mask: (/16) 255.255.0.0

First
non
-
255 number is 0

256


0 = 256

256 /
8
= 32 (that’s the magic number)


Mask: (/20) 255.255.240.0

First
non
-
255 number is
240

256


240
=
16

16
/ 8 =
2
(that’s the magic number
)

Numerical Calculations

On the 4
th

octet

On the 3
rd

octet

On the 3
rd

octet

© 2011 Cisco and/or its affiliates. All rights reserved.

7


Required number of subnets
12 (should be really looking at 16
)


Mask: (/25) 255.255.255.128

First non
-
255 number is 128

256


128 = 128

128 / 16 = 8 (that’s the magic number)


Mask: (/20) 255.255.240.0

First
non
-
255 number is
240

256


240
=
16

16
/
16
=
1
(that’s the magic number)


Mask: (/21) 255.255.248.0

First
non
-
255 number is
248

256


248
=
8

8

/
16
=
0.5 * 256 = 128
(that’s the magic number
)

Numerical Calculations

On the 4
th

octet

On the 3
rd

octet

Started on the 3
rd

but moved to the 4
th

octet

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Simply add the magic
nb

to the initial network. Example: subnetting
192.168.252.0 255.255.252.0 (/22) into 2, 4 or 8 subnets:



Numerical Calculations

2 Subnets

4 Subnets

8 Subnets

Magic

Nb =

(256
-
252)/2 = 2

Magic

Nb =

(256
-
252)/4 = 1

Magic Nb

=

(256
-
252)/8 = 128
(multiplied by 256)

192.168.252.0

192.168.252.0

192.168.252.0

192.168.254.0

192.168.253.0

192.168.252.128

192.168.254.0

192.168.253.0

192.168.255.0

192.168.253.128

192.168.254.0

192.168.254.128

192.168.255.0

192.168.255.128

© 2011 Cisco and/or its affiliates. All rights reserved.

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Subtract the magic number from 256. So to continue with the previous
example of subnetting 192.168.252.0/22:

Numerical Calculations

2 Subnets

4 Subnets

8 Subnets

Magic

Nb = 2 (3
rd

octet)

Magic

Nb = 1 (3
rd

octet)

Magic Nb

= 128 (4
th

octet)

256


2 = 254

256



1 = 255

256


128 =

128

255.255.254.0

(/23)

255.255.255.0 (/24)

255.255.255.128 (/25)

© 2011 Cisco and/or its affiliates. All rights reserved.

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With a subnet mask of 255.255.255.224, all network IDs would be a
multiple of (256


224 = 32), 4
th

octet, so
x.x.x.0
, .32, .64, …, .224



With a subnet mask of 255.255.128.0, all network IDs would be a
multiple of (256


128 = 128), 3
rd

octet, so
x.x.0.0
, .128.0

Numerical Calculations

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Subnet 192.168.16.0/20 into networks of 4 subnets



/20 => 255.255.240.0 => 256


240 = 16


16 / 4 = 4 (magic number, on the 3
rd

octet)


Networks are: 192.168.16.0, 192.168.20.0, 192.168.24.0,192.168.28.0


Mask is 256


4 = 252 => 255.255.252.0 or /22

Numerical Calculations

© 2011 Cisco and/or its affiliates. All rights reserved.

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1
st

and foremost: add the 2
IPs

of network ID and broadcast address


Look for the closest
exponentional

multiple of 2


That’s your network size.


Continue as previously explained.

Numerical Calculations

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Subnet 192.168.16.0/20 into networks of 700 hosts per subnet



700 => 702 => 1024 (network size)


1024 / 256 = 4 (magic number, on the 3
rd

octet)


Networks are: 192.168.16.0, 192.168.20.0, 192.168.24.0,192.168.28.0


Mask is 256


4 = 252 => 255.255.252.0 or /22

Numerical Calculations

© 2011 Cisco and/or its affiliates. All rights reserved.

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123.45.164.255/22
: network, broadcast or valid host? If broadcast or
valid host address, what is the network address
?



100.198.7.64/18
: network, broadcast or valid host? If broadcast or valid
host address, what is the network address
?



Hint
: from prefix, get mask, then get magic number, then see the closest
multiple of that number and
match it to
the appropriate value in the
address
provided above.



Numerical Calculations

© 2011 Cisco and/or its affiliates. All rights reserved.

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It gets a little bit confusing when it is requested to subnet a network into
several subnets, each with a different size.



So, let’s start with an example!

Numerical Calculations

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192.168.1.0/24 to be subnetted into A (100 hosts), B (40 hosts), C (10
hosts), D (2 hosts).










Remember to always start with the largest network first!


Numerical Calculations

Subnet

Nb of
Hosts

Net
Size

Magic
Nb

Mask

(4
th

octet)

Prefix

Subnet address

A

100

128

128

256
-
128=128

/25

192.168.1.0

B

40

64

64

256
-
64=192

/26

192.168.1.(0+128=128)

C

10

16

16

256
-
16=240

/28

192.168.1.(128+64=192)

D

2

4

4

256
-
4=252

/30

192.168.1.(192+16=208)

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For subnet A, start from the initial network address (192.168.1.0). The
next network would be a multiple of
the magic
nb
,
as explained before.
So, 192.168.1.128. Remember that we’re working on the 4th octet;


For subnet B, the address is 192.168.1.128. The next network would be
a multiple
of
the magic
nb
,
so we add 64 to 128. So, the next network is:
192.168.1.192.


For subnet C, address is 192.168.1.192. For the next network, we add
the
magic
nb

(16
) to the current address, so 192.168.1.208.


For subnet D, address is 192.168.1.208.


The
next network and all remaining addresses
are free



(192.168.1.212


192.168.1.255
)

Numerical Calculations

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192.168.160.0/19
to be subnetted into A (2000 hosts), B (500 hosts), C
(100 hosts), D (4
hosts).










Free addresses:
192.168.170.(128+8)


192.168.191.255.


Numerical Calculations

Subnet

Nb of
Hosts

Net
Size

Magic
Nb

Mask

Prefix

Subnet address

A

2000

2048

2048/256=8

256
-
8=248

(3
rd

octet)

/21

192.168.160.0

B

500

512

512/256=2

256
-
2=254

(3
rd

octet)

/23

192.168.(160+8=168).0

C

100

128

128

256
-
128=128

(4
th

octet)

/25

192.168.(168+2=170).0

D

4

8

8

256
-
8=248

(4
th

octet)

/29

192.168.170.(0+128)

© 2011 Cisco and/or its affiliates. All rights reserved.

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Magic Number


section end

Questions?

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Graphically
map
addresses
of an
octet


Very
easy to operate within
one octet


Rule of thumb: always divide in
half


Will ensure that you won’t waste/forget networks



Step 1: identify your
magic numbers (already know how to do it)


Step 2:
Segment and reserve
networks on the
bargraph

starting with the
largest network first
.


Step 3: You’re done!



Graphical Representation

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The
bargraph

shows a complete octet (from 0 till
255), so 256 values


You are only allowed to split a segment in half


Golden rule still applies: always start with the
largest network first


Choose the subnet locations according to needs


Graphical Representation

128

64

32

16

8

4

0

256

© 2011 Cisco and/or its affiliates. All rights reserved.

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Subnet 192.168.10.0/24 into:



A (100 hosts), B (20 hosts), C (10 hosts)



Graphical Representation

A (128)

B (32)

C (16)

FREE
Addresses






Width of 128







Width of 64



Width of 32


Width of 16


Subnet

Magic Nb

Subnet

ID

A

128

192.168.10.0 /25

B

32

192.168.10.128 /27

C

16

192.168.10.160

/28

FREE

192.168.10.176





192.168.10.255

Subnet

Magic Nb

A

128

B

32

C

16












Width of 256


This represents the 4
th

octet







0

128

160

176

255

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Graphical representation


section end

Questions?

Thank you.