This article explains how IP Prefix Size, amount of IP addresses in IP subnets, IP netmask and IP Classes are interrelated and correlate with each other. All arranged in one simple table.
Prefix table
Prefix size | Amount of IP addresses | Usable IP addresses | Amount of Class C address space | Netmask | Class |
/32 | 1 | 0 | 1/256 | 255.255.255.255 | Class D |
/31 | 2 | 0 | 1/128 | 255.255.255.254 | Class D |
/30 | 4 | 2 | 1/64 | 255.255.255.252 | Class D |
/29 | 8 | 6 | 1/32 | 255.255.255.248 | Class D |
/28 | 16 | 14 | 1/16 | 255.255.255.240 | Class D |
/27 | 32 | 30 | 1/8 | 255.255.255.224 | Class D |
/26 | 64 | 62 | 1/4 | 255.255.255.192 | Class D |
/25 | 128 | 126 | 1/2 | 255.255.255.128 | Class D |
/24 | 256 | 254 | 1 | 255.255.255.0 | Class C |
/23 | 512 | 510 | 2 | 255.255.254.0 | Class C |
/22 | 1024 | 1022 | 4 | 255.255.252.0 | Class C |
/21 | 2048 | 2046 | 8 | 255.255.248.0 | Class C |
/20 | 4096 | 4094 | 16 | 255.255.240.0 | Class C |
/19 | 8192 | 8190 | 32 | 255.255.224.0 | Class C |
/18 | 16384 | 16382 | 64 | 255.255.192.0 | Class C |
/17 | 32768 | 32766 | 128 | 255.255.128.0 | Class C |
/16 | 65536 | 65534 | 256 | 255.255.0.0 | Class C |
/15 | 131072 | 131070 | 512 | 255.254.0.0 | Class B |
/14 | 262144 | 262142 | 1024 | 255.252.0.0 | Class B |
/13 | 524288 | 524286 | 2048 | 255.248.0.0 | Class B |
/12 | 1048576 | 1048574 | 4096 | 255.240.0.0 | Class B |
/11 | 2097152 | 2097150 | 8192 | 255.224.0.0 | Class B |
/10 | 4194304 | 4194302 | 16384 | 255.192.0.0 | Class B |
/9 | 8388608 | 8388606 | 32768 | 255.128.0.0 | Class B |
/8 | 16777216 | 16777214 | 65536 | 255.0.0.0 | Class B |
/7 | 33554432 | 33554430 | 131072 | 254.0.0.0 | Class A |
/6 | 67108864 | 67108862 | 262144 | 252.0.0.0 | Class A |
/5 | 134217728 | 134217726 | 524288 | 248.0.0.0 | Class A |
/4 | 268435456 | 268435454 | 1048576 | 240.0.0.0 | Class A |
/3 | 536870912 | 536870910 | 2097152 | 224.0.0.0 | Class A |
/2 | 1073741824 | 1073741822 | 4194304 | 192.0.0.0 | Class A |
/1 | 2147483648 | 2147483648 | 8388608 | 128.0.0.0 | Class A |
NOTE: Keep in mind that in real life use cases, subnets need to have at least 4 IP addresses. This is because the first and the last IP address in a proper subnet is reserved for the gateway and broadcast respectively.