# Fast and Easy Subnetting

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24 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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Fast and Easy Subnetting

Marc Khayat, Technical Advocacy Team

makhayat@cisco.com

May 2011

2

Number of required subnets

Number of required hosts per subnet

3

Using the numerical calculations

Using the graphical representation

4

Network size is the number of IP addresses in a subnet, including the

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

5

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.
nb
!

Numerical Calculations

6

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

First non
-
255 number is 0

256

0 = 256

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

First
non
-
255 number is 0

256

0 = 256

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

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

7

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

First non
-
255 number is 128

256

128 = 128

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

First
non
-
255 number is
240

256

240
=
16

16
/
16
=
1
(that’s the magic number)

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

8

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

9

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)

10

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

11

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

4 = 252 => 255.255.252.0 or /22

Numerical Calculations

12

1
st

and foremost: add the 2
IPs

Look for the closest
exponentional

multiple of 2

That’s your network size.

Continue as previously explained.

Numerical Calculations

13

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

4 = 252 => 255.255.252.0 or /22

Numerical Calculations

14

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
provided above.

Numerical Calculations

15

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

Numerical Calculations

16

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

(4
th

octet)

Prefix

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)

17

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

18

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

192.168.170.(128+8)

192.168.191.255.

Numerical Calculations

Subnet

Nb of
Hosts

Net
Size

Magic
Nb

Prefix

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)

19

Magic Number

section end

Questions?

20

Graphically
map
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

21

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

22

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

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