Subnetting

The netmask is a bitmask that can be used to separate the bits of the network identifier from the bits of the host identifier. It is often written in the same notation used to denote IP addresses.

Not all sizes of prefix announcement may be routable on the public Internet: see routing, peering.

Class	Leading bits	Start	End	Default Subnet Mask in dotted decimal	CIDR notation
A	0	0.0.0.0	127.255.255.255	255.0.0.0	/8
B	10	128.0.0.0	191.255.255.255	255.255.0.0	/16
C	110	192.0.0.0	223.255.255.255	255.255.255.0	/24
D	1110	224.0.0.0	239.255.255.255	not defined	not defined
E	1111	240.0.0.0	255.255.255.254	not defined	not defined

The blocks numerically at the start and end of classes A, B and C were originally reserved for special addressing or future features, i.e., 0.0.0.0/8 and 127.0.0.0/8 are reserved in former class A; 128.0.0.0/16 and 191.255.0.0/16 are reserved in former class B; 192.0.0.0/24 and 223.255.255.0/24 are reserved in former class C.

While the 127.0.0.0/8 network is a Class A network, it is designated for loopback and cannot be assigned to a network.

CIDR	host bits	Netmask	Hosts in subnet	Classful name	Typical usage
/8	24	255.0.0.0	16777216 = 224	Class A	Largest block allocation made by IANA
/9	23	255.128.0.0	8388608 = 223
/10	22	255.192.0.0	4194304 = 222
/11	21	255.224.0.0	2097152 = 221
/12	20	255.240.0.0	1048576 = 220
/13	19	255.248.0.0	524288 = 219
/14	18	255.252.0.0	262144 = 218
/15	17	255.254.0.0	131072 = 217
/16	16	255.255.0.0	65536 = 216	Class B
/17	15	255.255.128.0	32768 = 215		ISP / large business
/18	14	255.255.192.0	16384 = 214		ISP / large business
/19	13	255.255.224.0	8192 = 213		ISP / large business
/20	12	255.255.240.0	4096 = 212		Small ISP / large business
/21	11	255.255.248.0	2048 = 211		Small ISP / large business
/22	10	255.255.252.0	1024 = 210
/23	9	255.255.254.0	512 = 29
/24	8	255.255.255.0	256 = 28	Class C	Large LAN
/25	7	255.255.255.128	128 = 27		Large LAN
/26	6	255.255.255.192	64 = 26		Small LAN
/27	5	255.255.255.224	32 = 25		Small LAN
/28	4	255.255.255.240	16 = 24		Small LAN
/29	3	255.255.255.248	8 = 23		Smallest multi-host network
/30	2	255.255.255.252	4 = 22		"Glue network" (point to point links)
/31	1	255.255.255.254	2 = 21		Rarely used, point to point links (RFC 3021)
/32	0	255.255.255.255	1 = 20		Host route

For example IP class C, this lesson got from my lecture Mr. Wagito (thanks Sir). Guess, the IP 192.168.112.55 is suitable for:

a. 192.168.112.0 /24
b. 192.168.112.0 /25
c. 192.168.112.0 /26
d. 192.168.112.0 /27

From the picture below whose created by Mr. Wagito, we can more understand that Ip 192.168.112.55 not include for 192.168.112.0 /27, because range for /27 just from 0 until 31.



Reference:
http://en.wikipedia.org/wiki/IPv4_subnetting_reference