CIDR 子网掩码和反掩码
/30 255.255.255.252 0.0.0.3
/29 255.255.255.248 0.0.0.7
/28 255.255.255.240 0.0.0.15
/27 255.255.255.224 0.0.0.31
/26 255.255.255.192 0.0.0.63
/25 255.255.255.128 0.0.0.127
/24 255.255.255.0 0.0.0.255
/23 255.255.254.0 0.0.1.255
/22 255.255.252.0 0.0.3.255
/21 255.255.248.0 0.0.7.255
/20 255.255.240.0 0.0.15.255
/19 255.255.224.0 0.0.31.255
/18 255.255.192.0 0.0.63.255
/17 255.255.128.0 0.0.127.255
/16 255.255.0.0 0.0.255.255
/15 255.254.0.0 0.1.255.255
/14 255.252.0.0 0.3.255.255
/13 255.248.0.0 0.7.255.255
/12 255.240.0.0 0.15.255.255
/11 255.224.0.0 0.31.255.255
/10 255.192.0.0 0.63.255.255
/9 255.128.0.0 0.127.255.255
/8 255.0.0.0 0.255.255.255