http://zh.wikipedia.org/wiki/CIDR Block | Hosts | Netmask | Amount of a Class C | /30 | 4 | 255.255.255.252 | 1/64 | /29 | 8 | 255.255.255.248 | 1/32 | /28 | 16 | 255.255.255.240 | 1/16 | /27 | 32 | 255.255.255.224 | 1/8 | /26 | 64 | 255.255.255.192 | 1/4 | /25 | 128 | 255.255.255.128 | 1/2 | /24 | 256 | 255.255.255.0 | 1 | /23 | 512 | 255.255.254.0 | 2 | /22 | 1024 | 255.255.252.0 | 4 | /21 | 2048 | 255.255.248.0 | 8 | /20 | 4096 | 255.255.240.0 | 16 | /19 | 8192 | 255.255.224.0 | 32 | /18 | 16384 | 255.255.192.0 | 64 | /17 | 32768 | 255.255.128.0 | 128 | /16 | 65536 | 255.255.0.0 | 256 | |