CIDR decimal number of addresses classful subnets
/1 128.0.0.0 2147483646 128 A 2
/2 192.0.0.0 1073741822 64 A 4
/3 224.0.0.0 536870910 32 A 8
/4 240.0.0.0 268435454 16 A 16
/5 248.0.0.0 134217726 8 A 32
/6 252.0.0.0 67108862 4 A 64
/7 254.0.0.0 33554430 2 A 128
/8 255.0.0.0 16777214 1 A 256
/9 255.128.0.0 8388606 128 B 512
/10 255.192.0.0 4194302 64 B 1024
/11 255.224.0.0 2097150 32 B 2048
/12 255.240.0.0 1048574 16 B 4096
/13 255.248.0.0 524286 8 B 8192
/14 255.252.0.0 262142 4 B 16384
/15 255.254.0.0 131070 2 B 32768
/16 255.255.0.0 65534 1 B 65536
/17 255.255.128.0 32766 128 C 131072
/18 255.255.192.0 16382 64 C 262144
/19 255.255.224.0 8190 32 C 524288
/20 255.255.240.0 4094 16 C 1048576
/21 255.255.248.0 2046 8 C 2097152
/22 255.255.252.0 1022 4 C 4194304
/23 255.255.254.0 510 2 C 8388608
/24 255.255.255.0 254 1 C 16777216
/25 255.255.255.128 126 1/2 C 33554432
/26 255.255.255.192 62 1/4 C 67108864
/27 255.255.255.224 30 1/8 C 134217728
/28 255.255.255.240 14 1/16 C 268435456
/29 255.255.255.248 6 1/32 C 536870912
/30 255.255.255.252 2 1/64 C 1073741824
/31 255.255.255.254 0 1/128 C  
/32 255.255.255.255 1    

What is a subnet mask?

Subnet mask tell us which part of an address is a network part and which is a host part. Network part designates a subnet and is used for routing to this subnet. Host part designates all members of this subnet and is useful only in this subnet. Subnet mask in IPv4 consist of 32 bits and normally is represented using dot-decimal form. Valid mask has ones from left side followed by zeros (after first zero can be only zeros). Example of a subnet mask is 255.255.255.0.