The Journey of a Network Engineer

Feb 22 2011   1:04AM GMT

How to subnet?



Posted by: Sulaiman Syed
Tags:
binary
CCNA
cheat sheet
Cisco
mathematics
subnet
subnetting

For any Network Engineer, knowing how to subnet is as important as typing ” en ” in the CLI. The engineer will have to type access lists that have wild masks, break networks to smaller parts, create vlans, point to point links, and design a whole new enterprise network. Subnetting is part of all these process.

I think most of the books will show how to subnet using the binary mathematics. which is the fundamental of IP addressing scheme. But is there any other way to do subnetting, without doing binary mathematics? The answer is yes.

Im not sure if this is a unique way that i have discovered, or someone else have been using it. A cheat sheet was made by me helped me to do all subnettings without touching a calculator. it did require doing some mathematics initially in constructing the cheat sheet, but once done no mathematics was required.

4th byte

25

26

27

28

29

30

31

32

3rd byte

17

18

19

20

21

22

23

24

2nd byte

9

10

11

12

13

14

15

16

1st byte

1

2

3

4

5

6

7

8

Mask

128

192

224

240

248

252

254

255

address

0

0

0

0

0

0

0

0

Magic number

128

64

32

16

8

4

2

1

128

64

32

16

8

4

2

192

96

48

24

12

6

3

128

64

32

16

8

4

160

80

40

20

10

5

192

96

48

24

12

6

224

112

56

28

14

7

128

64

32

16

8

144

.

36

18

9

160

.

40

20

10

176

.

.

22

11

192

.

.

24

12

208

.

.

26

13

224

.

.

.

14

240

.

.

.

.

248

.

.

.

252

.

.

254

.

255

Ok, so the table is not as complicated as it may seem. the First four rows indicate subnet mask, if it is numbered then you would need to convert it into decimal by referring to the table it gets clear. 5th raw is the subnet in decimal format. 6th row is where the network address will start. 7th raw is the magic number. the networks in these columns will always be multiplications of the magic number.

Taking a random example of 10.14.0.4/20. First, We find 20 in the first four rows. it is the 3rd byte with 240. and multiplication of 16. the ip address 10.14.0.4 comes between 0 and 16. so the network address is 10.14.0.0/20, broadcast is 10.14.15.255 (10.14.16.0 – 1).

another example 200.32.172.100/18. First, We find 18 in the first four rows. it is the 3rd byte with 192. and multiplication of 64. the ip address 200.32.172.100 comes between 128 and 192. so the network address is 200.32.128.0, broadcast is 200.32.191.255 (200.32.192.0 – 1).

This cheat sheet, helped me pass my CCNA exams. took the first 10 mins of the exam to make this cheat sheet. and then any question of subnetting was solved within a min or less.

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