Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses (see IP address allocation) and routing IP traffic that replaced the older classful network addressing system. Defined in RFC 1519, CIDR was introduced in 1993 to improve the scalability and efficiency of the IPv4 address space. By eliminating the fixed class structure (A, B, and C), CIDR allows for more flexible subnetting and aggregation of IP addresses, which helps optimize the allocation of IP addresses and reduces the size of routing tables on the internet.

In the older classful system, networks were assigned a fixed block of IP addresses based on the class of the address. Class A networks had a large number of available host addresses but were limited in the number of network identifiers, while Class C networks had many more network identifiers but fewer available host addresses. This system led to inefficiencies, with many organizations receiving far more addresses than they needed, contributing to the rapid exhaustion of the IPv4 address pool. CIDR solved this problem by allowing network addresses to be assigned with variable-length subnet masks (VLSM), giving organizations just the right number of addresses to meet their needs.

The key feature of CIDR is its use of the prefix length, which specifies how many bits of an IP address are used for the network portion. For example, in a CIDR notation such as 192.168.0.0/24, the “/24” represents the prefix length, meaning that the first 24 bits of the IP address represent the network portion, while the remaining 8 bits can be used to assign individual host addresses. This flexible approach allows networks to be divided into subnets of varying sizes, ensuring that IP addresses are allocated more efficiently.

CIDR also plays a crucial role in route aggregation, which allows routers to combine multiple IP addresses or subnets into a single routing entry. This process, known as supernetting, reduces the size of routing tables by summarizing multiple network addresses under a single prefix. For example, instead of listing multiple routes for networks like 192.168.0.0/24, 192.168.1.0/24, and 192.168.2.0/24, routers can aggregate these into a single route, such as 192.168.0.0/22. This aggregation reduces the complexity of routing and improves the scalability of internet routing systems.

Before the introduction of CIDR, routing tables grew rapidly as the internet expanded, threatening the stability and performance of the global internet. CIDR's ability to perform route aggregation significantly mitigated this problem, allowing the internet to continue growing without overwhelming routers with massive routing tables. This route aggregation also made it easier for internet service providers (ISPs) to manage and advertise their network blocks, reducing fragmentation in the global routing system.

In addition to improving address allocation and routing efficiency, CIDR also helped delay the exhaustion of the IPv4 address space. By allowing more granular allocation of addresses, CIDR reduced waste and extended the availability of IPv4 addresses. This was particularly important in the early 1990s when the demand for IP addresses was growing rapidly due to the expansion of internet-connected devices and networks.

The introduction of CIDR also brought changes to the way IP addresses are represented. In classful networking, the subnet mask was implicitly defined by the address class, but in CIDR, the subnet mask is explicitly defined using CIDR notation. For example, an address like 10.0.0.0/8 indicates that the first 8 bits of the IP address represent the network portion, while 10.0.0.0/16 would indicate a smaller subnet within the same range. This flexibility allows network administrators to create custom subnets that meet specific requirements for network size and address allocation.

While CIDR was developed primarily for IPv4, its principles are also used in IPv6, where the much larger address space allows for even more efficient address allocation. In IPv6, CIDR notation remains an essential part of network addressing, with prefix lengths commonly used to define subnet boundaries. For instance, an IPv6 address like 2001:db8::/32 would use a 32-bit prefix length to define the network portion.

CIDR has been a foundational technology in the growth and management of the modern internet. Its introduction allowed network operators to better utilize the available IPv4 address space and reduced the strain on routing infrastructure. Although the transition to IPv6 is ongoing, CIDR continues to play a critical role in ensuring the efficient management of both IPv4 and IPv6 address spaces.

Despite the widespread use of CIDR, the limitations of IPv4's address space have persisted, necessitating the deployment of IPv6 to address the long-term challenges of internet growth. However, CIDR's contribution to improving the scalability and efficiency of IPv4 routing and addressing is undeniable, and it remains a fundamental concept in both internet infrastructure and networking education.

Classless Inter-Domain Routing (CIDR), defined in RFC 1519, revolutionized IP address allocation and routing by eliminating the rigid class structure of classful networking and introducing variable-length subnet masks. CIDR improved the efficiency of IPv4 address usage, enabled route aggregation, and reduced the size of routing tables, addressing the challenges of rapid internet growth. While it continues to play a key role in IPv4 and IPv6 address management, CIDR has also been instrumental in ensuring the scalability and stability of the global internet.