What is CIDR and how can it be useful?

If you are involved in IP networks, you are likely familiar with the term CIDR. In this article, we will not only provide a definition of CIDR but also explore its practical applications and benefits. Having a clear understanding of CIDR can greatly assist in tasks related to the transfer of IPv4 and IP addresses in general.

What does CIDR stand for?

CIDR, which stands for Classless Inter-Domain Routing, is a replacement for the traditional classful IPv4 routing and address allocation policies. It is an IP addressing scheme that offers improved allocation of IPv4 addresses.

How does CIDR work?

In the past, IPv4 addresses were divided into three classes: A, B, and C, with each class having a fixed number of addresses. However, this system proved to be inefficient and led to address shortages. CIDR was introduced as a solution to this problem by allowing blocks of variable size instead of fixed classes. The CIDR notation represents an IP address and its associated network mask in a compact form. It consists of the IP address followed by a slash character (/) and a decimal number representing the size of the subnet mask. The subnet mask can range from /0 to /32, covering the entire range of possible addresses in an IPv4 address. This flexibility enabled the description of IPv4 blocks of any size without the limitations of the three class sizes. For example, a class C block, which originally contained 256 addresses, is represented in CIDR notation as /24, indicating that 24 bits of the address are masked, leaving 8 bits for addressing within the block (which corresponds to 256 possible numbers in binary). Similarly, a class A block is represented as /8, leaving 24 bits for addressing (equivalent to 16 million possible addresses in binary).

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CIDR Chart

Prefix /24Subnet Mask 256

Prefix /23Subnet Mask 512

Prefix /22Subnet Mask 1,024

Prefix /21Subnet Mask 2,048

Prefix /20Subnet Mask 4,096

Prefix /19Subnet Mask 8,192

Prefix /18Subnet Mask 16,384

Prefix /17Subnet Mask 32,768

Prefix /16Subnet Mask 65,536

Prefix /15Subnet Mask 131,072

Prefix /14Subnet Mask 262,144

Prefix /13Subnet Mask 524,288

Prefix /12Subnet Mask 1,048,576

Prefix /11Subnet Mask 2,097,152

Prefix /10Subnet Mask 4,194,304

Evgeny Sevastyanov

Client Support Teamleader