Subnetting Notes


Boolean Logic


0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1

0 OR 0 = 0
0 OR 1 = 1
1 OR 0 = 1
1 OR 1 = 1


Useful Graph


Magic number

128

64

32

16

8

4

2

1

CIDR bits

1

2

3

4

5

6

7

8

CIDR bits

9

10

11

12

13

14

15

16

CIDR bits

17

18

19

20

21

22

23

24

CIDR bits

25

26

27

28

29

30

31

32

Subnet mask

128

192

224

240

248

252

254

255


This graph helps me reduce the number of errors I make while answering subnettting questions on exams. I typically draw it as I sit down to an exam and then use it as a reference while checking my work before submitting an answer.


Find the column with the corresponding CIDR bits as the practice question you are working on and then pay attention to how that correlates to the "Magic number" and "Subnet mask" rows.

Subnetting Question


What are the subnet and broadcast IPs for host 10.19.213.6/20?


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