Net mask prefix lenght

Netmask     Address Prefix Length     Hosts / Class C's / Class B's / Class A's
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 (Class C)     256 / 1
255.255.254.0     /23     512 / 2
255.255.252.0     /22     1,024 / 4
255.255.248.0     /21     2,048 / 8
255.255.240.0     /20     4,096 / 16
255.255.224.0     /19     8,192 / 32
255.255.192.0     /18     16,384 / 64
255.255.128.0     /17     32,768 / 128
255.255.0.0     /16 (Class B)     65,536 / 256 / 1
255.254.0.0     /15     131,072 / 512 / 2
255.252.0.0     /14     262,144 / 1024 / 4
255.248.0.0     /13     524,288 / 2048 / 8
255.240.0.0     /12     1,048,576 / 4096 / 16
255.224.0.0     /11     2,097,152 / 8129 / 32
255.192.0.0     /10     4,194,304 / 16,384 / 64
255.128.0.0     /9     8,388,608 / 32,768 / 128
255.0.0.0     /8 (Class A)     16,777,216 / 65,536 / 256 / 1
254.0.0.0     /7     33,554,432 / 131,072 / 512 / 2
252.0.0.0     /6     67,108,864 / 262,144 / 1,024 / 4
248.0.0.0     /5     134,217,728 / 524,288 / 2,048 / 8
240.0.0.0     /4     268,435,456 / 1,048,576 / 4,096 / 16
224.0.0.0     /3     536,870,912 / 2,097,152 / 8,192 / 32
192.0.0.0     /2     1,073,741,824 / 4,194,304 / 16,384 / 64
128.0.0.0     /1     2,147,483,648 / 8,388,608 / 32,768 / 128
0.0.0.0     /0 (The Internet)     4,294,967,296 / 16,777,216 / 65,536 / 256