Classless Interdomain Routing

Definition of Classless Interdomain Routing

Classless Interdomain Routing (CIDR) is a network routing method that replaces the traditional class-based IP addressing system. It allows for more efficient allocation and use of IP addresses by using variable-length subnet masking (VLSM) to divide the IP address space into smaller, flexible network blocks. CIDR helps reduce IP address wastage and mitigate the depletion of available IPv4 addresses.


The phonetics of the keyword “Classless Interdomain Routing” are as follows:Classless: K-L-A-S-L-E-SInterdomain: I-N-T-ER-D-O-M-A-I-NRouting: R-O-U-T-I-N-G

Key Takeaways

  1. Classless Interdomain Routing (CIDR) is a scalable and efficient IP addressing scheme that aims to reduce the exhaustion of IPv4 addresses by eliminating the rigid class-based structure and introducing Variable Length Subnet Masking (VLSM).
  2. CIDR helps in aggregating IP address blocks, which simplifies routing table updates and minimizes the space required for routing information, resulting in better overall performance of the Internet routing system.
  3. In CIDR notation, the IP address and the subnet mask are combined into a single string with a forward slash (/) separating the address and the subnet mask length. The format is ‘IP address/subnet mask length’, e.g.,, making subnetting more efficient and flexible.

Importance of Classless Interdomain Routing

Classless Interdomain Routing (CIDR) is an essential technology term in networking as it plays a significant role in the efficient allocation and management of IP addresses.

CIDR allows for flexible and hierarchical IP address assignment, which in turn helps mitigate the exhaustion of IPv4 addresses by enabling route aggregation and minimizing the size of routing tables.

This reduced routing table size optimizes the routing process, resulting in faster lookups and improved overall network performance.

Additionally, the CIDR approach supports both IPv4 and IPv6 addressing schemes, ensuring its continued relevance as networks transition towards future IP-based communication systems.


Classless Interdomain Routing (CIDR) is a pivotal innovation in Internet Protocol (IP) address assignment that was developed to manage the scarcity of IP addresses and improve their efficiency within routing procedures. CIDR’s primary purpose is to optimize the allocation of IP addresses and facilitate the efficient routing of data packets in a global internet environment.

Prior to CIDR, IP addresses were categorized into fixed classes (A, B, and C) which restrained efficient utilization due to their predetermined size limitations. The introduction of CIDR, however, allowed network administrators to allocate IP addresses in a more context-specific manner, adapting to an organization’s needs by utilizing a Variable-Length Subnet Mask (VLSM).CIDR achieves its objectives by utilizing a methodology that combines an IP address and its corresponding subnet mask into a singular notation, eliminating the dependence on rigid class-based structures.

This allows for addressing schemes that are better suited to the size and requirements of individual networks, which can result in reduced waste of address space and more effective use of available IP ranges. Furthermore, CIDR contributes to improving the scalability and performance of internet routing by aggregating multiple routes into a single, more manageable entity.

This process, known as route aggregation or route summarization, simplifies routing tables and minimizes the burden on routers, consequently accelerating data packet transmission and enhancing overall efficiency in the internet ecosystem.

Examples of Classless Interdomain Routing

Classless Interdomain Routing (CIDR) is a vital technology utilized in Internet Protocol (IP) networks to facilitate efficient IP address allocation and minimize the number of routing entries a router needs. Here are three real-world examples of CIDR in action:

Networking in large organizations: Large corporations, universities, or government organizations with thousands of computers and servers need an effective IP addressing scheme. CIDR enables such organizations to efficiently allocate their IP addresses by dividing their IP address space into smaller, variable-sized blocks called subnetworks or subnets. This helps optimize the routing of data packets and reduces the burden on routers.

Internet Service Providers (ISPs): ISPs manage a large pool of IP addresses for their customers. CIDR helps in aggregating multiple smaller address blocks into a single, larger address block, thus optimizing the IP address management process. Utilizing CIDR, ISPs can efficiently allocate blocks of IP addresses to their subscribers and reduce the size of global routing tables, improving overall performance and efficiency of the Internet.

Content Distribution Networks (CDNs): CDNs are networks designed to distribute web content across geographically diverse data centers, reducing latency and improving website performance for users. CIDR is an essential technology in managing and routing data among CDN servers. By adopting CIDR, CDNs can optimize the routing of data packets and efficiently direct users to the closest server to their locations, ensuring quick content delivery.In all these examples, the adoption of CIDR has contributed to better performance, efficiency, and scalability when managing IP addresses and routing data across complex networks.

Classless Interdomain Routing

What is Classless Interdomain Routing (CIDR)?

Classless Interdomain Routing (CIDR) is a method for allocating IP addresses and routing Internet Protocol packets. It replaces the older system based on classes A, B, and C and allows more efficient allocation of IP addresses, helping to slow down the exhaustion of IPv4 address space.

How does CIDR work?

CIDR groups IP addresses into blocks known as CIDR blocks, which are identified by a prefix and a subnet mask. This allows Internet Service Providers (ISPs) to allocate IP addresses more efficiently since they can distribute IP ranges in different sizes according to the needs of their clients.

What is the difference between CIDR and traditional IP classes?

Traditional IP classes (A, B, and C) divided the IP address space into fixed-sized blocks, each allocated to different organizations. This led to inefficiencies and wasted address space, particularly for organizations that required more IP addresses than the blocks allowed. CIDR doesn’t use fixed-sized blocks. Instead, it allows for variable-length subnet masks, leading to more efficient allocation of IP addresses.

How is a CIDR notation written?

CIDR notation is written in the format “IP_address/prefix_length”. The IP address represents the first address in the block, and the prefix length indicates the number of consecutive 1 bits in the subnet mask. For example, represents the IP address block to, with a subnet mask of (24 consecutive 1 bits followed by 8 consecutive 0 bits).

What is the main advantage of CIDR over traditional IP classes?

The main advantage of CIDR over traditional IP classes is its efficient use of IP addresses. By using variable-length subnet masks, CIDR allows organizations to request the exact number of IP addresses they require, reducing the amount of wasted address space and delaying the exhaustion of IPv4 address space. Additionally, CIDR simplifies routing tables by aggregating multiple smaller networks into a single entry.

Related Technology Terms

  • IP Addressing
  • Subnet Mask
  • Route Aggregation
  • Variable Length Subnet Mask (VLSM)
  • Border Gateway Protocol (BGP)

Sources for More Information


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