CIDR (Classless inter-domain routing)

CIDR (Classless inter-domain routing) is a method of public IP address assignment. It was introduced in 1993 by Internet Engineering Task Force with the following goals:

  • to deal with the IPv4 address exhaustion problem
  • to slow down the growth of routing tables on Internet routers

Before CIDR, public IP addresses were assigned based on the class boundaries:

  • Class A – the classful subnet mask is /8. The number of possible IP addresses is 16,777,216 (2 to the power of 24).
  • Class B – the classful subnet mask is /16. The number of addresses is 65,536
  • Class C – the classful subnet mask is /24. Only 256 addresses available.

Some organizations were known to have gotten an entire Class A public IP address (for example, IBM got all the addresses in the range). Since these addresses can’t be assigned to other companies, there was a shortage of available IPv4 addresses. Also, since IBM probably didn’t need more than 16 million IP addresses, a lot of addresses were unused.

To combat this, the classful network scheme of allocating the IP address was abandoned. The new system was classsless – a classful network was split into multiple smaller networks. For example, if a company needs 12 public IP addresses, it would get something like this:

The number of usable IP addresses can be calculated with the following formula:

2 to the power of host bits – 2

In the example above, the company got 14 usable IP addresses from the – range because there are 4 host bits and 2 to the power of 4 minus 2 is 14 The first and the last address are the network address and the broadcast address,,respectively. All other addresses inside the range could be assigned to Internet hosts.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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