# Bits # Hosts Netmask Cisco Mask Notes
/4 268435456 240.0.0.0 15.255.255.255
/5 134217728 248.0.0.0 7.255.255.255
/6 67108864 252.0.0.0 3.255.255.255
/7 33554432 254.0.0.0 1.255.255.255
/8 16777216 255.0.0.0 0.255.255.255 class A network
/9 8388608 255.128.0.0 0.127.255.255
/10 4194304 255.192.0.0 0.63.255.255
/11 2097152 255.224.0.0 0.31.255.255
/12 1048576 255.240.0.0 0.15.255.255
/13 524288 255.248.0.0 0.7.255.255
/14 262144 255.252.0.0 0.3.255.255
/15 131072 255.254.0.0 0.1.255.255
/16 65536 255.255.0.0 0.0.255.255 class B network
/17 32768 255.255.128.0 0.0.127.255
/18 16384 255.255.192.0 0.0.63.255
/19 8192 255.255.224.0 0.0.31.255
/20 4096 255.255.240.0 0.0.15.255
/21 2048 255.255.248.0 0.0.7.255
/22 1024 255.255.252.0 0.0.3.255
/23 512 255.255.254.0 0.0.1.255
/24 256 255.255.255.0 0.0.0.255 class C network
/25 128 255.255.255.128 0.0.0.127
/26 64 255.255.255.192 0.0.0.63
/27 32 255.255.255.224 0.0.0.31
/28 16 255.255.255.240 0.0.0.15
/29 8 255.255.255.248 0.0.0.7
/30 4 255.255.255.252 0.0.0.3
/31 point to point links only
/32 1 255.255.255.255 single IP address use host notation

Note:
The number of usable hosts is always two less than the overall number of hosts in the sub-network.

Other nice source: https://www.aelius.com/njh/subnet_sheet.html