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.

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