Column
Value/Increment
(Magic Number)
Bits Borrowed
Number of
Subnets
1st Octet CIDR
2nd Octet CIDR
3rd Octet CIDR
4th Octet CIDR
128
64
32
16
8
4
2
1
1
2
2
4
3
8
4
16
5
32
6
64
7
128
8
256
/1
/9
/17
/25
/2
/10
/18
/26
/3
/11
/19
/27
/4
/12
/20
/28
/5
/13
/21
/29
/6
/14
/22
/30
/7
/15
/23
/31
/8
/16
/24
/32
255.0.0.0 =>
/8 =>
11111111.00000000.00000000.00000000
255.255.0.0 =>
/16 => 11111111.11111111.00000000.00000000
255.255.255.0 =>
/24 => 11111111.11111111.11111111.00000000
Octet Values for Subnet Mask
0
00000000
128
10000000
192
11000000
224
11100000
240
11110000
248
11111000
252
11111100
254
11111110
255
11111111
2S =
Number of subnets a mask will yield; where “S” is the number of bits borrowed
from the host.
2H – 2 = Number of hosts a mask will yield; where “H” is the number of binary zeroes in
the subnet mask.