If you are in the business of IPv4 and IP Addresses you’ve probably heard or seen the term CIDR, but do you know what it is? Knowing what CIDR stands for is helpful, but it’s even better to explore how CIDR works and understand the concept. Below are some frequently asked questions that will increase your understanding of the term and why it is important.

What does CIDR Stand for?

CIDR (pronounced like “cider”) stands for “Classless Inter-Domain Routing”. It replaced the original “classful” IPv4 address routing and allocation policies. CIDR is an IP Addressing scheme that improves the allocation of IP addresses, by allowing subnets of IPv4 ranges to be announced over the Internet  in a flexible array of sizes.

Why is CIDR Important?

In the beginning, IPv4 blocks were designated in classes: A, B, and C. Class A had roughly 16 million addresses, Class B had roughly 65 thousand, and Class C had 256 addresses. This lack of granularity led to inefficient allocations and usage of increasingly scarce IPv4 addresses.   For instance, if you were an organization allocated a Class B IPv4 block prior to CIDR, you had to announce the entire block of 65,536 addresses in one chunk.  You couldn’t subdivide the block and announce portions of it from different Internet peering locations. 

CIDR replaced classes with a nomenclature allowing for variable sized blocks, using an appellation called a subnet mask, designated as the number of masked bits behind a slash. Since the total number of bits in an IPv4 address is 32 bits, the size of the subnet mask can vary from a /0 (the whole internet) to a /32 (a single IPv4 address).  This allowed for allocations and routing entries to describe any size of IPv4 block without the classful limit of only three sizes.

The original Class C, the smallest class containing just 256 addresses, is written as a /24 in CIDR notation. That means out of the total 32 bits of address space, 24 bits are masked, leaving only 8 bits of address space in the block. In binary terms, 8 bits equals 256 possible numbers. Likewise, a Class A network is now written as a /8, leaving 24 bits of address space in the block. Again, in binary terms 24 bits yields 16 million addresses.

Benefits of CIDR Notations

The benefits are clear when considering an entity with a need for 500 addresses. In the past, this would be larger than a Class C, and would require allocation of a Class B network containing 65,636 addresses. With CIDR, the allocation can be a /23, twice as large as a /24 and providing 512 addresses.   So instead of wasting an entire Class B (a /16), less than 1% of the Class B is allocated, leaving the remainder available.

CIDR’s introduction in the late 1990s was the largest reason for the extension of viable life for IPv4, which was anticipated to be exhausted in just a few years under the classful allocation regime. These days, the block sizes associated with particular subnet mask sizes are usually memorized by brokers with years of experience:

CIDR Chart

/24  256

/23  512

/22 1,024

/21 2,048

/20 4,096

/19 8,192

/18 16,384

/17 32,768

/16 65,536

/15 131,072

/14  262,144

/13 524,288

/12 1,048,576

/11 2,097,152

/10 4,194,304

/0  4,294,967,296  (the whole Internet!)

And so forth.  Likewise, network engineers are quite familiar with CIDR notation and the subnets it creates. CIDR allows subnets as small as a /24 (256 addresses) to be partitioned and broadcast from much larger blocks. While Internet routers do not support BGP broadcasts of subnets smaller than a /24, ISPs with direct connections to their clients can route smaller blocks (such as a /27 with 32 addresses), allowing for even more efficient IPv4 allocations.