An Example of CIDR in Use in the Case Study

Imagine that the company discussed in the case study at the beginning of the chapter, Mental Merge, has applied for a Class B address from the Internet authorities. To everyone's surprise, Mental Merge has been awarded seven Class C networks. The company owners are delighted because they were expecting only one Class C address.

Figure 3-3 shows the addresses awarded to Mental Merge, the use of CIDR addresses, and how prefix routing works at the binary level.

Figure 3-3 Prefix Routing and the Use of CIDR

Entire IP Address


Class C address -Supernet Address 110010000.01100100.00110

000 001 010 011 100 101 110 111

.00000000 .00000000 .00000000 .00000000 .00000000 .00000000 .00000000 .00000000

Although eight Class C addresses are provided to the organization, they are identified to the Internet as one address:, with a prefix mask of /21, which is the subnet mask of

The organization does not have to use the addresses as Class C addresses. In accordance with the original rules, the organization may use the right-most zeroed bits however they deem appropriate.

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