Classless Inter-Domain Routing (CIDR) is more efficient than the older classful network design for several key reasons;

1. Flexible Subnetting and Address Allocation

Classful Network Design:

  • IP addresses were divided into fixed classes (A, B, C, D, E).
  • Class A: 16 million addresses (8-bit network prefix).
  • Class B: 65,536 addresses (16-bit network prefix).
  • Class C: 256 addresses (24-bit network prefix).
  • This fixed structure often led to significant wastage of IP addresses. For example, a network needing 300 addresses would require a Class B allocation, wasting most of the 65,536 addresses.


  • Allows for arbitrary-length network prefixes, enabling more precise allocation of IP addresses.
  • Example: A CIDR block like can allocate 1,024 addresses (covering to, which is more efficient than allocating a larger block unnecessarily.
  • This flexibility reduces wastage and allows for better utilization of IP address space.

2. Simplified Routing and Reduced Routing Table Size

Classful Network Design

  • Each network class required its entry in routing tables.
  • The rigid class boundaries could lead to numerous routing table entries, especially with many small networks.


  • Enables route aggregation (supernetting), which combines multiple IP address ranges into a single routing table entry.
  • Example: Multiple networks like,,, and can be aggregated into a single CIDR block
  • This reduces the number of entries in routing tables, leading to faster and more efficient routing.

3. Improved Internet Scalability

Classful Network Design

  • The rapid growth of the Internet led to the exhaustion of available Class B and C addresses.
  • The class-based allocation system struggled to keep up with the expanding number of networks.


  • Delays IPv4 address exhaustion by allowing more granular allocation of address space.
  • Supports hierarchical IP address allocation, which improves the scalability of the global routing system.
  • Internet Service Providers (ISPs) can allocate IP addresses more efficiently, reducing the need for frequent renumbering and reallocation.

4. Enhanced Network Design Flexibility

Classful Network Design

  • Networks were constrained to predefined sizes, making it difficult to design networks that matched organizational needs precisely.
  • Limited flexibility in managing and optimizing network resources.


  • Provides the ability to create subnets and supernets tailored to specific requirements.
  • Organizations can design networks that closely match their size and growth expectations, optimizing resource usage and management.

Summary of Efficiency Gains

  • Address Utilization: CIDR reduces IP address wastage by allowing precise subnetting.
  • Routing Efficiency: CIDR minimizes routing table size through route aggregation, enhancing routing performance.
  • Scalability: CIDR supports the growth of the Internet by enabling more efficient use of the IPv4 address space.
  • Flexibility: CIDR offers greater flexibility in network design, accommodating diverse organizational needs.

Practical Example

Imagine a company needing IP addresses for 500 hosts. Under the classful system, they would have to use a Class B network (/16), which provides 65,536 addresses, wasting a vast majority. With CIDR, they can allocate a 23-bit network prefix (e.g.,, providing 512 addresses, which is a much more efficient use of IP space.

By adopting CIDR, the efficiency of IP address allocation and routing is significantly improved, making it a preferred choice for modern networking.


IP Address Guide provides detailed explanations and tools for understanding and calculating IP addresses and subnets.

RFC 1519 is the original document that specifies CIDR.

RFC 1918 discusses private IP address allocation, which is crucial for understanding network prefixes in private networks.

IETF publishes a wide range of RFCs and documents related to internet standards, including those on IP networking and CIDR.

Classless Inter-Domain Routing

Network Computing offers a variety of articles and tutorials on networking topics.

Cisco Documentation provides detailed technical guides and white papers on various networking topics, including IP addressing and subnetting.

Juniper Networks offers comprehensive technical documentation on IP networking and related topics.

IEEE Xplore is a digital library for research papers and articles on electrical engineering and computer science, including networking topics.

ACM Digital Library provides access to a vast collection of research articles and papers on computer science, including networking.


  1. “Computer Networks” by Andrew S. Tanenbaum and David J. Wetherall

    • A comprehensive textbook covering a wide range of topics in computer networking, including IP addressing and subnetting.
  2. “TCP/IP Illustrated, Volume 1: The Protocols” by W. Richard Stevens

    • A detailed guide to the TCP/IP protocol suite, providing in-depth coverage of IP networking concepts.
  3. “Internetworking with TCP/IP Volume One” by Douglas E. Comer

    • Another authoritative resource that explains the principles of TCP/IP networking, including IP addressing and CIDR.