Classless Inter-Domain Routing (CIDR)

Classless Inter-Domain Routing (CIDR)

InterNIC no longer gives out addresses under the Class A, B, or C designations. Instead, it uses a method called Classless Inter-Domain Routing (or CIDR, which is usually pronounced “cider”).
CIDR networks are described as “slash x (/x)” networks; the x represents the number of bits in the IP address range that InterNIC controls, more easily seen as the number of bits in the subnet mask
set to 1. This allows InterNIC to define networks that fall between the old classifications, which means that you can get a range of addresses much better suited to your needs than in times past. In
CIDR terms, a network classified as a Class C network under the old scheme becomes a /24 network because InterNIC controls the leftmost 24 bits and you control the rightmost 8 bits. Table 3.5 shows some examples of /x network types.

Note:

You can also combine multiple classful networks into a single network using
this same designation system. This process is known as supernetting.

TABLE 3 . 5 Table 3.5Examples of CIDR Network Types