CIDR stands for, Classless Inter Domain Routing. CIDR is a way to allocate and specify the Internet addresses used in inter-domain router more flexibly than with the original system of Internet Protocol address classes. As a result, the number of available Internet addresses has been greatly increased. CIDR is now the routing system used by virtually all gateway hosts on the Internet's backbone network. Many small companies also benefit from CIDR in that 4 Class C's can be grouped into one subnet to provide over 1000 IP Addresses.

----------------------------------------------------
Network Type Subnet Mask No of Ips
----------------------------------------------------
Slash 0 | 0.0.0.0 | 4 billion
Slash 1 | 128.0.0.0 | 2 billion
Slash 2 | 192.0.0.0 | 1billion
Slash 3 | 224.0.0.0 | 500 million
Slash 4 | 240.0.0.0 | 250 million
Slash 5 | 248.0.0.0 | 128 million
Slash 6 | 252.0.0.0 | 64 million
Slash 7 | 254.0.0.0 | 32 million
Slash 8 | 255.0.0.0 | 16 million
Slash 9 | 255.128.0.0 | 8 million
Slash 10 | 255.192.0.0 | 4 million
Slash 11 | 255.224.0.0 | 2 million
Slash 12 | 255.240.0.0 | 1 million
Slash 13 | 255.248.0.0 | 524288
Slash 14 | 255.252.0.0 | 262144
Slash 15 | 255.254.0.0 | 131072
Slash 16 | 255.255.0.0 | 65536
Slash 17 | 255.255.128.0 | 32768
Slash 18 | 255.255.192.0 | 16384
Slash 19 | 255.255.224.0 | 8192
Slash 20 | 255.255.240.0 | 4096
Slash 21 | 255.255.248.0 | 2048
Slash 22 | 255.255.252.0 | 1024
Slash 23 | 255.255.254.0 | 512
Slash 24 | 255.255.255.0 | 256
Slash 25 | 255.255.255.128 | 128
Slash 26 | 255.255.255.192 | 64
Slash 27 | 255.255.255.224 | 32
Slash 28 | 255.255.255.240 | 16
Slash 29 | 255.255.255.248 | 8
Slash 30 | 255.255.255.252 | 4
Slash 31 | 255.255.255.254 | 2
Slash 32 | 255.255.255.255 | 1

The first few on the list would be very difficult to manage network and the last two meaningless subnet mask that would serve no purpose since the only two ip addresses available are taken by the either the network number and broadcast address or are lacking in either.

sumber : http://www.netdummy.net/cidr.html