The following table provides a convenient shortcut to all the CIDR prefix
sizes, showing the number of addresses possible in each prefix and the
number of prefixes of that size that may be numbered in the 32-bit IPv4
address space:

notation       addrs/block      # blocks
--------       -----------     ----------
n.n.n.n/32               1     4294967296    "host route"
n.n.n.x/31               2     2147483648    "p2p link"
n.n.n.x/30               4     1073741824
n.n.n.x/29               8      536870912
n.n.n.x/28              16      268435456
n.n.n.x/27              32      134217728
n.n.n.x/26              64       67108864
n.n.n.x/25             128       33554432
n.n.n.0/24             256       16777216    legacy "Class C"
n.n.x.0/23             512        8388608
n.n.x.0/22            1024        4194304
n.n.x.0/21            2048        2097152
n.n.x.0/20            4096        1048576
n.n.x.0/19            8192         524288
n.n.x.0/18           16384         262144
n.n.x.0/17           32768         131072
n.n.0.0/16           65536          65536    legacy "Class B"
n.x.0.0/15          131072          32768
n.x.0.0/14          262144          16384
n.x.0.0/13          524288           8192
n.x.0.0/12         1048576           4096
n.x.0.0/11         2097152           2048
n.x.0.0/10         4194304           1024
n.x.0.0/9          8388608            512
n.0.0.0/8         16777216            256    legacy "Class A"
x.0.0.0/7         33554432            128
x.0.0.0/6         67108864             64
x.0.0.0/5        134217728             32
x.0.0.0/4        268435456             16
x.0.0.0/3        536870912              8
x.0.0.0/2       1073741824              4
x.0.0.0/1       2147483648              2
0.0.0.0/0       4294967296              1    "default route"

n is an 8-bit decimal octet value.

x is a 1- to 7-bit value, based on the prefix length, shifted into the most
significant bits of the octet and converted into decimal form; the least
significant bits of the octet are zero.

Source: http://www.bgp4.as/