Short Form, Full Form, Maximum number of machines, Comment
/8 /255.0.0.0 16, 777, 215 Class A address
/16 /255.255.0.0 65, 535 Class B address
/17 /255.255.128.0 32, 767
/18 /255.255.192.0 16, 383
/19 /255.255.224.0 8, 191
/20 /255.255.240.0 4, 095
/21 /255.255.248.0 2, 047
/22 /255.255.252.0 1, 023
/23 /255.255.254.0 511
/24 /255.255.255.0 255 Class C address
/25 /255.255.255.128 127
/26 /255.255.255.192 63
/27 /255.255.255.224 31
/28 /255.255.255.240 15
/29 /255.255.255.248 7
/30 /255.255.255.252 3

ref: Network Masks / Netmasks for Dummies