What is Classless Inter-Domain Routing?

In the evolving world of networking, efficient use of IP addresses has become increasingly important. Enter CIDR—Classless Inter-Domain Routing—a method that transformed the way we allocate and manage IP addresses. This guide will take you on a deep dive into CIDR, explaining its origins, benefits, and how it works in today’s networks.

What is CIDR?

CIDR stands for Classless Inter-Domain Routing. It was introduced in the 1990s as a solution to the rapid depletion of IPv4 addresses and to improve the efficiency of routing on the internet. Unlike the older classful addressing system (Classes A, B, and C), CIDR allows for a more flexible allocation of IP addresses.

Classful vs. Classless

In classful addressing, IP addresses are divided into fixed blocks (classes) with predetermined sizes. This rigidity often led to wasted IP addresses. CIDR, on the other hand, allows networks to be divided into blocks of varying sizes based on actual needs—making the system more scalable and efficient.

How Does CIDR Work?

CIDR notation is written as an IP address followed by a slash and a number (e.g., 192.168.1.0/24). The number after the slash (called the prefix) tells you how many bits are used for the network portion of the address. Since IPv4 addresses are 32 bits long, the number of bits left for host addresses is:

Host bits = 32 − prefix

For example:

/24: Uses 24 bits for the network, leaving 32 − 24 = 8 bits for hosts.
/30: Uses 30 bits for the network, leaving 32 − 30 = 2 bits for hosts.

Because each bit can be either 0 or 1, the total number of IP addresses in the block is:

Why Use 32? Not 31 or 30?

IPv4 addresses are defined as 32-bit numbers. That’s why our calculations always use 32 as the starting point:

32 Bits Total: The entire IPv4 address space is based on 32 bits, which provides about 4.3 billion unique addresses.
Network vs. Host Division: The CIDR prefix tells you how many of those bits are fixed for the network, leaving the remaining bits for individual device addresses.

Changing the 32 to 31 or 30 would ignore the standard definition of IPv4 and would not correctly represent the available address space.

How Many IP Addresses Do You Get?

When you see a CIDR notation like /30 or /29, here’s how you calculate the total number of addresses:

Quick Reference Table

Note: In many practical scenarios, two addresses are reserved in each block—the network address and the broadcast address—so the number of usable addresses is slightly less than the total calculated.

The Benefits of CIDR

CIDR offers several advantages over the old class-based system:

Efficient Use of Address Space:
By allowing variable-length subnet masks, CIDR reduces wasted addresses. You can tailor the block size to exactly what you need.
Improved Routing Efficiency:
CIDR enables route aggregation (or supernetting), where multiple contiguous IP blocks are combined into one routing entry. This makes routing tables smaller and more efficient.
Simplified Network Management:
Network design networks that match the actual number of devices, whether it's a small home network or a large enterprise system.

Real-World Applications

Internet Service Providers (ISPs):
ISPs use CIDR to allocate and aggregate IP address blocks, making network routing more efficient.
Corporate Networks:
Companies use CIDR to divide large networks into smaller, manageable sub-networks, ensuring security and efficient communication between departments.
Cloud and Data Centers:
With frequent changes in virtual machines and containers, CIDR allows for flexible and dynamic IP allocation.

Conclusion

CIDR has revolutionized IP addressing by introducing flexibility, efficiency, and scalability. With its simple notation, you can quickly determine the number of IP addresses in any block and tailor your network to meet your exact needs. Understanding CIDR is a crucial step for anyone interested in networking, cybersecurity, or just curious about how the internet works.

Happy networking, and may your digital pathways always be clear and well-addressed!