IP地址 CIDR / 子网掩码 /8 - 255.0.0.0 /16 - 255.255.0.0 /20 - 255.255.240.0 /21 - 255.255.248.0 /22 - 255.255.252.0 /23 - 255.255.254.0 /24 - 255.255.255.0 /25 - 255.255.255.128 /26 - 255.255.255.192 /27 - 255.255.255.224 /28 - 255.255.255.240 /29 - 255.255.255.248 /30 - 255.255.255.252