Classless Inter-Domain Routing (CIDR)
Another term you need to familiarize yourself with is Classless Inter-Domain Routing
(CIDR). It’s basically the method that Internet service providers (ISPs) use to allocate a
number of addresses to a company, a home—their customers. They provide addresses in a
certain block size, something I’ll talk about in greater detail soon.
When you receive a block of addresses from an ISP, what you get will look something
like this: 192.168.10.32/28. This is telling you what your subnet mask is. The slash notation
(/) means how many bits are turned on (1s). Obviously, the maximum could only be
/32 because a byte is 8 bits and there are 4 bytes in an IP address: (4 n 8 = 32). But keep in
mind that regardless of the class of address, the largest subnet mask available relevant to
the Cisco exam objectives can only be a /30 because you’ve got to keep at least 2 bits for
host bits.

Take, for example, a Class A default subnet mask, which is 255.0.0.0. This tells us that
the first byte of the subnet mask is all ones (1s), or 11111111. When referring to a slash
notation, you need to count all the 1-bits to figure out your mask. The 255.0.0.0 is considered
a /8 because it has 8 bits that are 1s—that is, 8 bits that are turned on.
A Class B default mask would be 255.255.0.0, which is a /16 because 16 bits are ones
(1s): 11111111.11111111.00000000.00000000.

Source: www.google.com