A quick reference table for subnet masks that you can check at a glance.
| IP Count (Host Count + 2) | Mask Value | Mask Length |
| 1 | 255.255.255.255 | 32 |
| 2 | 255.255.255.254 | 31 |
| 4 | 255.255.255.252 | 30 |
| 8 | 255.255.255.248 | 29 |
| 16 | 255.255.255.240 | 28 |
| 32 | 255.255.255.224 | 27 |
| 64 | 255.255.255.192 | 26 |
| 128 | 255.255.255.128 | 25 |
| 256 | 255.255.255.0 | 24 |
| 512 | 255.255.254.0 | 23 |
| 1024 | 255.255.252.0 | 22 |
| 2048 | 255.255.248.0 | 21 |
| 4096 | 255.255.240.0 | 20 |
| 8192 | 255.255.224.0 | 19 |
| 16384 | 255.255.192.0 | 18 |
| 32768 | 255.255.128.0 | 17 |
| 65536 | 255.255.0.0 | 16 |
| 131072 | 255.254.0.0 | 15 |
| 262144 | 255.252.0.0 | 14 |
| 524288 | 255.248.0.0 | 13 |
| 1048576 | 255.240.0.0 | 12 |
| 2097152 | 255.224.0.0 | 11 |
| 4194304 | 255.192.0.0 | 10 |
| 8388608 | 255.128.0.0 | 9 |
| 16777216 | 255.0.0.0 | 8 |
| 33554432 | 254.0.0.0 | 7 |
| 67108864 | 252.0.0.0 | 6 |
| 134217728 | 248.0.0.0 | 5 |
| 268435456 | 240.0.0.0 | 4 |
| 536870912 | 224.0.0.0 | 3 |
| 1073741824 | 192.0.0.0 | 2 |
| 2147483648 | 128.0.0.0 | 1 |
| 4294967296 | 0.0.0.0 | 0 |
・Subnet Mask Calculation Table
That’s all from the Gemba.