Table 2. Netmask and VLSM Notation

Note that since we're dealing with 32 numbers, we can count the number of ones from the left and use that as a shorthand for addressing:

255.255.0.0     = /16
255.255.128.0   = /17
255.255.255.0   = /24
255.255.255.128 = /25
255.255.255.192 = /26
255.255.255.224 = /27
255.255.255.240 = /28