Have you ever wondered what is a /27?  Or how many IP addresses do I get to use with a /28?  Is there a use for a /31? 

CIDR

Subnet Mask

Total IPs

Usable IPs

/32
255.255.255.255
1
1

/31
255.255.255.254
2
0*

/30
255.255.255.252
4
2

/29
255.255.255.248
8
6

/28
255.255.255.240
16
14

/27
255.255.255.224
32
30

/26
255.255.255.192
64
62

/25
255.255.255.128
128
126

/24
255.255.255.0
256
254

/23
255.255.254.0
512
510

/22
255.255.252.0
1024
1022

/21
255.255.248.0
2048
2046

/20
255.255.240.0
4096
4094

/19
255.255.224.0
8192
8190

/18
255.255.192.0
16,384
16,382

/17
255.255.128.0
32,768
32,766

/16
255.255.0.0
65,536
65,534

/15
255.254.0.0
131,072
131,070

/14
255.252.0.0
262,144
262,142

/13
255.248.0.0
524,288
524,286

/12
255.240.0.0
1,048,576
1,048,574

/11
255.224.0.0
2,097,152
2,097,150

/10
255.192.0.0
4,194,304
4,194,302

/9
255.128.0.0
8,388,608
8,388,606

/8
255.0.0.0
16,777,216
16,777,214

/7
254.0.0.0
33,554,432
33,554,430

/6
252.0.0.0
67,108,864
67,108,862

/5
248.0.0.0
134,217,728
134,217,726

/4
240.0.0.0
268,435,456
268,435,454

/3
224.0.0.0
536,870,912
536,870,910

/2
192.0.0.0
1,073,741,824
1,073,741,822

/1
128.0.0.0
2,147,483,648
2,147,483,646

/0
0.0.0.0
4,294,967,296
4,294,967,294

* /31 networks can be used for point-to-point connections.  Your devices must support this ability.