You can block /8 (16,777,216 IPs) though I would recommend /16 (65,536 IPs) or less unless you are trying to block an entire country and know that the entire /8 range is exclusive to that country.
The correct way to block full networks is with zeros in the IP: 192.0.0.0/16 (192 is the first three of the IP network you want to block.
|
CIDR |
Subnet Mask |
Total IPs |
Usable IPs |
| /32 | 255.255.255.255 | 1 | 1 |
| /31 | 255.255.255.254 | 2 | 0 |
| /30 | 255.255.255.252 | 4 | 2 |
| /29 | 255.255.255.248 | 8 | 6 |
| /28 | 255.255.255.240 | 16 | 14 |
| /27 | 255.255.255.224 | 32 | 30 |
| /26 | 255.255.255.192 | 64 | 62 |
| /25 | 255.255.255.128 | 128 | 126 |
| /24 | 255.255.255.0 | 256 | 254 |
| /23 | 255.255.254.0 | 512 | 510 |
| /22 | 255.255.252.0 | 1024 | 1022 |
| /21 | 255.255.248.0 | 2048 | 2046 |
| /20 | 255.255.240.0 | 4096 | 4094 |
| /19 | 255.255.224.0 | 8192 | 8190 |
| /18 | 255.255.192.0 | 16,384 | 16,382 |
| /17 | 255.255.128.0 | 32,768 | 32,766 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /15 | 255.254.0.0 | 131,072 | 131,070 |
| /14 | 255.252.0.0 | 262,144 | 262,142 |
| /13 | 255.248.0.0 | 524,288 | 524,286 |
| /12 | 255.240.0.0 | 1,048,576 | 1,048,574 |
| /11 | 255.224.0.0 | 2,097,152 | 2,097,150 |
| /10 | 255.192.0.0 | 4,194,304 | 4,194,302 |
| /9 | 255.128.0.0 | 8,388,608 | 8,388,606 |
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
