CIDR Blocks

CIDR Explained

CIDR stands for “Classless Inter-Domain Routing.” It is a notation used in IP (Internet Protocol) addressing and routing. CIDR allows for a more flexible and efficient allocation of IP addresses and is used to represent IP address ranges and subnet masks. It enables network administrators to group IP addresses into larger or smaller blocks, making IP address allocation more efficient and adaptable.


The Usefulness of Classless Inter-Domain Routing

Classless Inter-Domain Routing (CIDR) is useful for several reasons:

  1. Efficient IP Address Allocation: CIDR allows for a more flexible allocation of IP addresses. Instead of being restricted to fixed and often inefficiently sized address blocks (as in traditional classful addressing), CIDR enables the allocation of variable-sized subnets, ensuring efficient utilization of address space.
  2. Conservation of IP Addresses: With the rapid growth of the internet and the depletion of IPv4 addresses, CIDR helps conserve IP addresses by allowing more precise assignment of address blocks based on actual network requirements. This reduces address wastage.
  3. Hierarchical Routing: CIDR facilitates hierarchical routing, where IP addresses are aggregated into larger prefixes. This simplifies routing tables in routers and improves the efficiency of internet routing.
  4. Flexible Subnetting: CIDR provides the ability to create subnets of different sizes, allowing network administrators to design networks that align with their organization’s specific needs, without being constrained by rigid classful boundaries.
  5. Simplified Addressing and Routing: CIDR eliminates the need to adhere to class boundaries (Class A, B, C, etc.), making IP addressing and routing more straightforward and less prone to errors.
  6. Internet Growth and Scalability: As the internet continues to grow, CIDR helps manage the increasing number of IP addresses more effectively, contributing to the overall scalability of the global network.
  7. Support for Variable-Length Subnet Masks (VLSM): CIDR enables the use of variable-length subnet masks, which allows for more efficient allocation of IP addresses within a subnet. This is especially valuable for optimizing address usage in larger networks.
  8. Easier Address Aggregation: CIDR enables the aggregation of multiple IP address ranges into a single routing entry. This reduces the size of routing tables and enhances routing efficiency.

Overall, CIDR offers greater flexibility, improved address allocation, simplified routing, and enhanced scalability in the management of IP addresses within modern networks and the internet.


The Creation of CIDR Blocks

CIDR blocks were created as a way to address the limitations of the traditional IP addressing scheme, which was based on classes. Classful IP addressing, introduced in the early days of the internet, had some inherent inefficiencies, such as wasting IP addresses and making it difficult to allocate addresses in a flexible manner. CIDR (Classless Inter-Domain Routing) was introduced to provide more efficient and flexible allocation of IP addresses.

CIDR blocks are created by aggregating and subdividing IP address ranges in a more efficient way than the classful system. Here’s how CIDR blocks are created:

  1. Aggregation of IP Addresses: CIDR allows IP addresses to be aggregated into larger blocks, reducing the number of routing table entries in routers. This aggregation helps improve the scalability and efficiency of routing on the internet.
  2. Variable-Length Subnet Masking (VLSM): In the traditional classful addressing, all subnets within a class had the same subnet mask. CIDR introduced the concept of VLSM, where different subnets could have different subnet masks. This allows for more efficient allocation of IP addresses, as you can allocate smaller subnets where needed and larger subnets where more addresses are required.
  3. Prefix Lengths: CIDR notation uses prefix lengths to define the number of significant bits in the subnet mask. For example, a /24 CIDR block means the first 24 bits of the IP address represent the network portion. This allows for fine-grained control over subnet sizes.
  4. Hierarchical Addressing: CIDR enables a hierarchical addressing structure, where IP address space can be divided into smaller and more manageable subnets. This hierarchical approach makes routing more efficient and helps conserve IP address space.
  5. Efficient Address Utilization: CIDR allows organizations to allocate IP addresses based on their actual needs, reducing address wastage. It also simplifies IP address management by providing a more flexible and granular way to allocate addresses.

Overall, CIDR was created to address the shortcomings of the classful IP addressing system and to provide a more scalable, efficient, and flexible way to allocate and manage IP addresses on the internet. It has played a significant role in the growth and management of the global IP address space.


How Many CIDR Blocks There Are

The number of possible CIDR blocks is virtually infinite due to the flexible nature of CIDR notation. CIDR notation allows for the creation of subnets with varying sizes, which means that the number of possible combinations is extensive.

In CIDR notation, the format typically follows an IP address followed by a slash (“/”) and then a number indicating the length of the prefix. The prefix length represents the number of bits in the subnet mask. For example, a /24 CIDR block corresponds to a subnet mask of, which means the first 24 bits are reserved for the network portion.

Given that the number after the slash can vary from 1 to 32 (for IPv4), there are 32 possible prefix lengths for each IP address. For each of these prefix lengths, you can create multiple CIDR blocks by varying the network portion of the address.

As an example, if you consider the IPv4 address, you can create multiple CIDR blocks such as:

  • and so on…

For each of these CIDR blocks, you have countless possibilities for IP addresses within that block.

In summary, the number of possible CIDR blocks is vast and dependent on the flexibility offered by the combination of IP addresses and various prefix lengths.

CIDR Conversion Table

CIDR Length Mask # of Networks #  of Hosts
/1 128 A 2,147,483,392
/2 64 A 1,073,741,696
/3 32 A 536,870,848
/4 16 A 268,435,424
/5 8 A 134,217,712
/6 4 A 67,108,856
/7 2 A 33,554,428
/8 1 A 16,777,214
/9 128 B 8,388,352
/10 64 B 4,194,176
/11 32 B 2,097,088
/12 16 B 1,048,544
/13 8 B 524,272
/14 4 B 262,136
/15 2 B 131,068
/16 1 B 65,024
/17 128 C 32,512
/18 64 C 16,256
/19 32 C 8,128
/20 16 C 4,064
/21 8 C 2,032
/22 4 C 1,016
/23 2 C 508
/24 1 C 254
/25 2 subnets 124
/26 4 subnets 62
/27 8 subnets 30
/28 16 subnets 14
/29 32 subnets 6
/30 64 subnets 2
/31 none none
/32 none 1

Helpful Links

CIDR Conversion Calculator

Subnet Calculator

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