Ccna routing and Switching Complete Study Guide

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 × 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.

Table 4.2 has a listing of every available subnet mask and its equivalent CIDR slash notation.

Ta b l E   4 . 2     CIDR values