The number of available host addresses in a subnet is calculated by the following forumula:
hosts = 2 ^ 32 - CIDR
For example, a /24 subnet has 2 ^ 8 = 256 hosts.
CIDR | Netmask | Hex | dotted quad | Hosts | Usable | Comment |
/32 | 1111111 11111111 11111111 11111111 | FFFFFFFF | 255.255.255.255 | 1 | 1 | single host |
/31 | 1111111 11111111 11111111 11111110 | FFFFFFFE | 255.255.255.254 | 2 | 2 | ACL for 2 hosts |
/30 | 1111111 11111111 11111111 11111100 | FFFFFFFC | 255.255.255.252 | 4 | 2 | usually a WAN or PPP address |
/29 | 1111111 11111111 11111111 11111000 | FFFFFFF8 | 255.255.255.248 | 8 | 6 | small static subnet |
/28 | 1111111 11111111 11111111 11110000 | FFFFFFF0 | 255.255.255.240 | 16 | 14 | subnet |
/27 | 1111111 11111111 11111111 11100000 | FFFFFFE0 | 255.255.255.224 | 32 | 30 | subnet |
/26 | 1111111 11111111 11111111 11000000 | FFFFFFC0 | 255.255.255.192 | 64 | 62 | subnet |
/25 | 1111111 11111111 11111111 10000000 | FFFFFF80 | 255.255.255.128 | 128 | 126 | subnet |
/24 | 1111111 11111111 11111111 00000000 | FFFFFF00 | 255.255.255.0 | 256 | 254 | previously Class C |
/23 | 1111111 11111111 11111110 00000000 | FFFFFE00 | 255.255.254.0 | 512 | 510 | supernet |
/22 | 1111111 11111111 11111100 00000000 | FFFFFC00 | 255.255.252.0 | 1024 | 1022 | supernet |
/21 | 1111111 11111111 11111000 00000000 | FFFFF800 | 255.255.248.0 | 2048 | 2046 | supernet |
/20 | 1111111 11111111 11110000 00000000 | FFFFF000 | 255.255.240.0 | 4096 | 4094 | supernet |
/19 | 1111111 11111111 11100000 00000000 | FFFFE000 | 255.255.224.0 | 8192 | 4090 | supernet |
/18 | 1111111 11111111 11000000 00000000 | FFFFC000 | 255.255.192.0 | 16384 | 16382 | supernet |
/17 | 1111111 11111111 10000000 00000000 | FFFF8000 | 255.255.128.0 | 32768 | 32766 | supernet |
/16 | 1111111 11111111 00000000 00000000 | FFFF0000 | 255.255.0.0 | 65536 | 65534 | previously Class B |