CIDR

What is CIDR?

Classless Inter-Domain Routing (CIDR) represents a set of IP standards aimed at enhancing the efficiency of IP address allocation and optimizing packet routing across networks. Known also as supernetting, CIDR was introduced in 1993 as a modern alternative to the older class-based addressing system. Under the traditional classful addressing scheme, internet routers classified and routed network traffic by categorizing IP addresses into three classes: A, B, and C, each with a predefined network size and a default subnet mask.

In stark contrast, CIDR employs variable-length subnet masking (VLSM), which divides the address space into subnets of varying sizes in a hierarchical fashion. This method detaches the IP address space from its default subnet mask, enabling the creation of subnetworks with diverse host counts and minimizing wasted address space. By resolving the scalability and inefficiency issues inherent in the previous class-based system, CIDR has played a crucial role in preventing the premature exhaustion of IPv4 addresses.

CIDR Notation Explained

Following its introduction by the Internet Engineering Task Force (IETF), network engineers needed a new way to manage and label IP addresses. CIDR notation emerged as a standardized method to address this need. It provides a compact and efficient means of representing IP addresses along with their associated network or subnet masks and the total number of bits in the address.

In CIDR notation, an IP address is expressed in the format w.x.y.z/n, where the suffix /n indicates the number of leading bits in the IP address. For example, in the address 214.100.48.0/20, the /20 suffix signifies a 20-bit network mask. This tersely specifies the network's size without detailing the entire subnet mask in dotted decimal format. CIDR notation applies to both IPv4 and IPv6 addresses, where IPv4 addresses are confined to 32 bits, and IPv6 addresses extend up to 128 bits. Numerous tools, such as CIDR calculators, are available to swiftly determine the IP range for a given CIDR address.

Understanding CIDR Blocks

CIDR represents IP address allocations and packet routing characteristics using leading bits and prefix-based standards. A CIDR block is a group of IP addresses that share the same prefix and vary in bit lengths. Larger CIDR blocks, indicated by shorter prefixes, can encompass more IP addresses, whereas smaller blocks, indicated by longer prefixes, contain fewer IP addresses. For instance, the block notation a.b.c.d/n uses the /n to represent the prefix length.

Combining multiple CIDR blocks with shared network prefixes to form larger routing networks is known as supernetting. A supernet is essentially a larger network segment containing adjacent CIDR blocks. Supernetting, also referred to as route aggregation or summarization, conserves address space and enhances network routing efficiency.

Managing and Assigning CIDR Blocks

The Internet Assigned Numbers Authority (IANA) oversees the distribution and management of CIDR blocks. Large blocks of IP addresses are allocated to Regional Internet Registries (RIRs), which serve extensive geographic regions such as Europe, Africa, and North America. These RIRs further divide the larger blocks into smaller subnets, eventually assigning them to Local Internet Registries (LIRs). This subdivision process continues until the IP addresses reach the end users.

The size of the assigned blocks varies based on the specific address needs of each end-user network. The IETF recommends that end-user networks sourcing IP addresses from a single Internet Service Provider (ISP) obtain their address space directly from the ISP. Conversely, networks utilizing multiple ISPs should procure provider-independent blocks from their respective LIR or RIR.

How CIDR Operates

CIDR operates based on classless addressing, a significant advancement over the traditional classful method which restricted prefix lengths and led to inefficient IP address allocation. The classful system, which divided address space into classes A, B, and C primarily, faced issues of address exhaustion as user requirements often necessitated rounding up to the next largest class. This resulted in wastage of address space.

In CIDR, the address space is divided using VLSM into subnets of varying sizes. A subnet is a logical or physical subdivision of a network, consisting of IP addresses with common routing prefixes. Subnetting reduces network congestion by minimizing bandwidth load on the network.

CIDR's subnet masking technique splits the IP address into host and network parts, where a subnet mask comprises a sequence of 1s (for the network) followed by 0s (for the host). This guides routers on which part of the IP address denotes the network and which part pertains to the hosts.

CIDR also allows the creation of supernets—combining consecutive networks into a single, larger network. This technique, known as route aggregation, consolidates route advertisements, allowing data packets to be routed efficiently to one destination regardless of the host location. This reduces the number of entries in the routing table and is particularly useful for organizations managing extensive networks.

Advantages of CIDR Notation

The CIDR notation offers numerous benefits over classful addressing, including:

• Optimized Address Allocation: By allowing IP addresses of varying sizes instead of fixed-size blocks, CIDR significantly reduces wasted IP addresses. It enables efficient and flexible assignment of address space according to the number of hosts.

• Reduced Routing Table Entries: CIDR minimizes routing table entries, enhancing the efficiency of packet routing. A single routing entry can represent multiple networks due to CIDR's hierarchical subnet structure.

• Simplified Notation: Compared to the dotted decimal notation, CIDR's format is more concise, representing the network mask with a single value. This streamlined approach is particularly useful for multi-segment displays.

Overall, CIDR has revolutionized IP address representation and allocation, making it a fundamental tool for modern network management.