Classless inter domain routing

What Is Classless Inter-Domain Routing (CIDR)?

Classless Inter-Domain Routing (CIDR) is a method for allocating Internet Protocol (IP) addresses and routing IP packets more efficiently. It was introduced to replace the previous classful network addressing system, which had become inefficient and was contributing to the rapid depletion of IPv4 addresses. CIDR falls under the broader category of Networking & Internet Protocols, addressing how network resources are managed and distributed. This system allows for greater flexibility in dividing the IP address space, enabling network administrators to assign address blocks that precisely fit the needs of organizations, rather than being restricted to predefined sizes. CIDR significantly improved the scalability and efficiency of the internet's network architecture by reducing the size and complexity of routing tables.

History and Origin

The initial design of the Internet Protocol, specifically IPv4, utilized a "classful" addressing system that categorized IP addresses into fixed-size blocks (Class A, B, and C). While simple, this approach led to significant inefficiencies in resource allocation. For instance, a Class B network, suitable for large organizations, provided over 65,000 unique addresses, which was often far more than needed, leading to substantial wastage of the finite IPv4 address space. Conversely, a Class C network, with only 256 addresses, was frequently too small. This rigidity, coupled with the explosive growth of the internet in the late 1980s and early 1990s, accelerated the depletion of available IPv4 addresses and caused an unsustainable expansion of global routing tables¹⁴.

To combat these looming issues, the Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing in 1993 through RFC 1519 and RFC 1518¹², ¹³. This development was crucial in delaying the complete exhaustion of IPv4 addresses and improving the scalability of internet routing. CIDR essentially removed the rigid class boundaries, allowing network prefixes to be defined at any bit boundary, a concept known as Variable-Length Subnet Masking (VLSM). This innovation provided much finer control over subnet sizes, enabling a more granular and efficient distribution of IP addresses. The formal allocation of the final IPv4 address blocks from the Internet Assigned Numbers Authority (IANA) to the regional Internet registries (RIRs) began in 2011, further highlighting the criticality of CIDR and the subsequent push for IPv6 adoption¹⁰, ¹¹.

Key Takeaways

• Efficient Address Allocation: CIDR allows for a more flexible and granular assignment of IP addresses, mitigating the waste associated with the rigid classful addressing system.
• Reduced Routing Table Size: By enabling the aggregation of multiple, smaller IP address blocks into larger "supernets," CIDR helps to compress routing table entries, making internet routing more efficient.
• CIDR Notation: IP addresses are expressed using a "slash notation" (e.g., 192.168.1.0/24), where the number after the slash indicates the length of the network prefix in bits.
• Delaying IPv4 Exhaustion: CIDR was a critical solution that significantly extended the lifespan of IPv4 by optimizing its usage until the broader adoption of IPv6.
• Foundation for Modern Networking: CIDR underpins modern network design, enabling more complex and efficient network topologies.

Formula and Calculation

CIDR does not involve a traditional financial formula but rather a method for interpreting and calculating network and host portions of an IP address. An IP address in CIDR notation is represented as:

IP_Address/Prefix_Length

Where:

• IP_Address is the standard 32-bit IPv4 address (or 128-bit IPv6 address).
• Prefix_Length (or n) is an integer from 0 to 32 (for IPv4) that indicates the number of bits in the network portion of the address. The remaining bits ($32 - n$ for IPv4) are used for the host portion.

To determine the number of assignable IP addresses within a CIDR block:

Number of Hosts = $2^{(32 - n)} - 2$

The subtraction of 2 accounts for the network address (all host bits set to 0) and the broadcast address (all host bits set to 1), which are reserved and cannot be assigned to individual devices. This concept is fundamental to subnetting.

Interpreting the CIDR

Interpreting a CIDR block involves understanding which part of the IP address identifies the network and which part identifies specific devices within that network. The Prefix_Length (the number following the slash) is the key. For example, in 192.168.1.0/24, the /24 indicates that the first 24 bits of the 32-bit IPv4 address define the network. This means that all IP addresses starting with 192.168.1. belong to the same network. The remaining 8 bits ($32 - 24 = 8$) are available for assigning to individual hosts or devices within that network.

A shorter prefix length indicates a larger network block, encompassing more IP addresses. Conversely, a longer prefix length signifies a smaller network block with fewer assignable IP addresses. For instance, a /16 block (e.g., 10.0.0.0/16) is much larger than a /24 block, allowing for more individual hosts or smaller subnets to be created within it. Understanding this interpretation is crucial for network administrators managing digital infrastructure and designing efficient IP addressing schemes.

Hypothetical Example

Imagine a growing technology startup, "InnovateTech," that initially used a classful addressing scheme and found it highly inefficient. They were assigned a Class C network block, 203.0.113.0, providing 254 usable IP addresses (203.0.113.1 to 203.0.113.254). As they grew, they needed to segment their network for different departments (e.g., Development, Marketing, Operations) and guest Wi-Fi, but the single Class C block was too restrictive for creating multiple, appropriately sized subnets.

With CIDR, InnovateTech's network administrator decides to use a larger block and subdivide it more flexibly. Suppose they acquire 192.168.50.0/22.

• Understanding the Block: The /22 means the first 22 bits are for the network address. This block covers IP addresses from 192.168.50.0 to 192.168.53.255.
• Subnetting with CIDR: The administrator can now create smaller subnets tailored to each department's needs:
  • Development: 192.168.50.0/24 (254 usable addresses)
  • Marketing: 192.168.51.0/24 (254 usable addresses)
  • Operations: 192.168.52.0/25 (126 usable addresses for a smaller team)
  • Guest Wi-Fi: 192.168.52.128/26 (62 usable addresses for temporary users)
  • Servers: 192.168.53.0/28 (14 usable addresses for critical servers)

This example shows how CIDR allows InnovateTech to precisely manage their data packets and allocate IP addresses without significant waste, optimizing their use of their assigned address space far more efficiently than the older, classful system. It enables granular control over network segments.

Practical Applications

Classless Inter-Domain Routing is fundamental to how the modern internet functions and is widely applied across various aspects of information technology and telecommunications:

• Internet Service Providers (ISPs): ISPs use CIDR to allocate IP address blocks to their customers efficiently. This allows them to manage their vast pool of addresses and distribute them in variable sizes, from large corporate allocations to smaller residential ones, optimizing their bandwidth and network capacity.
• Enterprise Networks: Large organizations leverage CIDR to design complex internal networks, segmenting them into numerous subnets for different departments, geographical locations, or specific services. This improves security, manages network traffic, and enhances overall network performance.
• Cloud Computing: Cloud providers like Amazon Web Services (AWS), Google Cloud, and Microsoft Azure rely heavily on CIDR for provisioning virtual private clouds (VPCs) and assigning IP address ranges to cloud resources. This allows customers to define their own network topologies within the cloud, ensuring isolation and control over their virtual networks.
• Routing Aggregation: CIDR facilitates route aggregation, also known as supernetting. This process combines multiple, smaller routing entries into a single, larger entry in internet routers, significantly reducing the size of global routing tables. This is crucial for maintaining the efficiency and speed of internet traffic flow⁹.
• Network Security: CIDR notation is used in firewall rules and access control lists (ACLs) to specify ranges of IP addresses that are permitted or denied access. This allows for precise control over network traffic and helps enforce network security policies.

Limitations and Criticisms

While Classless Inter-Domain Routing offers significant advantages, it also introduces certain complexities and potential drawbacks, particularly when compared to the simpler classful addressing system it replaced.

One primary limitation of CIDR is its increased complexity in network design and management. Unlike the fixed, easily recognizable boundaries of classful addressing, CIDR's flexible prefix lengths require a deeper understanding of binary mathematics and subnetting concepts. This can lead to configuration errors if administrators lack sufficient expertise, potentially causing connectivity issues or security vulnerabilities. Network administration becomes a more specialized task.

Compatibility issues can also arise with older networking hardware or software that might not fully support CIDR. While most modern equipment is CIDR-compliant, legacy systems might struggle to interpret CIDR blocks, leading to interoperability challenges during network upgrades or expansions⁸.

Furthermore, while CIDR significantly delayed IPv4 address exhaustion, it is not a permanent solution to the fundamental scarcity of 32-bit IP addresses. The rapid proliferation of internet-connected devices, driven by concepts like the Internet of Things (IoT), continues to put pressure on the remaining IPv4 address space. This ongoing demand necessitates the transition to IPv6, which offers a vastly larger address space⁷. Despite its drawbacks, CIDR remains a vital technology, but its inherent limitations underscore the long-term necessity of a complete shift to IPv6 for sustainable internet growth.

CIDR vs. Classful Addressing

The fundamental difference between Classless Inter-Domain Routing (CIDR) and traditional classful addressing lies in how IP addresses are divided into network and host portions.

Classful addressing, while simpler to conceptualize, was designed in an era when the internet's scale was unforeseen. It assigned IP address blocks based on predefined octet boundaries, leading to only three primary network sizes. This meant organizations often received address blocks far larger or smaller than their actual needs, causing immense waste of the limited IPv4 addresses⁵, ⁶.

CIDR, in contrast, decouples the network identifier from the octet boundaries by using a subnet mask of variable length, indicated by the /Prefix_Length notation. This allows network administrators to create subnetworks of virtually any size that is a power of two, from a single host to millions. The flexibility of CIDR directly addresses the inefficiencies of classful addressing by enabling more granular and efficient resource utilization and reducing the strain on global routing infrastructure⁴. The confusion between the two often arises from the continued use of "Class A, B, C" terminology even in CIDR contexts, though these now merely refer to the size of historical blocks rather than a strict addressing rule.

FAQs

What does the number after the slash mean in CIDR?

The number after the slash in CIDR notation (e.g., /24 in 192.168.1.0/24) indicates the "prefix length." This number specifies how many bits, starting from the left, are used to identify the network portion of the IP address. The remaining bits define the host portion within that network. For IPv4, the total number of bits is 32.

How does CIDR save IP addresses?

CIDR saves IP addresses by allowing network administrators to allocate address blocks more precisely. Instead of being forced to use large, fixed-size classful blocks (like Class B networks which wasted many addresses for smaller organizations), CIDR enables the creation of smaller, custom-sized blocks that match an organization's actual needs, thus reducing waste of the finite IPv4 address space², ³.

Is CIDR used with IPv6?

Yes, CIDR principles are fundamental to IPv6 address allocation and routing. While IPv6 has a much larger address space (128 bits compared to IPv4's 32 bits) and therefore doesn't face the same address exhaustion concerns, it still uses CIDR notation (e.g., 2001:db8::/32) to define network prefixes and enable efficient routing protocols. The concept of route summarization is equally important for managing IPv6 networks.

What is the smallest CIDR block?

For IPv4, the smallest practical CIDR block commonly used is a /30, which provides 4 total addresses (2 usable host addresses, plus the network and broadcast addresses). A /31 can be used for point-to-point links (2 addresses, both usable), and a /32 represents a single host IP address. For IPv6, the standard practice allocates larger minimum blocks, typically a /64 for individual subnets.

Does CIDR improve internet speed?

CIDR does not directly improve the speed of individual data transmissions. However, it significantly contributes to the overall efficiency and stability of the internet by reducing the size of routing tables. Smaller routing tables mean routers can process and forward internet traffic more quickly and with less computational overhead, which indirectly contributes to a more responsive internet¹.