有了這個表就不用自己算了
掩碼 位 IP數
255.255.255.255 32 1
255.255.255.254 31 2
255.255.255.252 30 4
255.255.255.248 29 8
255.255.255.240 28 16
255.255.255.224 27 32
255.255.255.192 26 64
255.255.255.128 25 128
255.255.255.0 24 256
255.255.254.0 23 512
255.255.252.0 22 1024
255.255.248.0 21 2048
255.255.240.0 20 4096
255.255.224.0 19 8192
255.255.192.0 18 16384
255.255.128.0 17 32768
255.255.0.0 16 65536
255.254.0.0 15 131072
255.252.0.0 14 262144
255.248.0.0 13 524288
255.240.0.0 12 1048576
255.224.0.0 11 2097152
255.192.0.0 10 4194304
255.128.0.0 9 8388608
255.0.0.0 8 16777216
254.0.0.0 7 33554432
252.0.0.0 6 67108864
248.0.0.0 5 134217728
240.0.0.0 4 268435456
224.0.0.0 3 536870912
192.0.0.0 2 1073741824
128.0.0.0 1 2147483648
0.0.0.0 0 4294967296