What Is CIDR?

• CIDR Definition

  CIDR Definition

  Classless Inter-Domain Routing (CIDR) is a set of IP standards designed to optimize the allocation of IP addresses by forming unique identifiers. This method for allocating IP addresses and routing IP packets was introduced in 1993 to replace the previous system based on address classes (Classes A, B, and C). The network size is fixed in classful addressing, and each address range comes with a default subnet mask.

  CIDR uses a technique called variable-length subnet masking (VLSM), which is why it is often referred to as supernetting or prefix aggregation. It involves splitting the address space.

  This division consists of various sizes of subnets arranged in a hierarchical manner. This decoupling of IP address space from a default subnet mask enables the creation of subnetworks with distinct host counts, thereby reducing wasted address space. Since its introduction, the CIDR addressing scheme has effectively handled the scalability and inefficiency constraints associated with the previous class-based system, preventing unnecessary exhaustion of IPV4  addresses.
• What Is CIDR Notation?

  What Is CIDR Notation?

  The CIDR notation is the compact syntax used to represent an IP address and its associated network mask (or network prefix length). It’s the standard method of writing IP network addresses today, replacing the old fixed-size classful system.

  After CIDR was introduced by the Internet Engineering Task Force (IETF), network engineers and technicians faced the challenge of tracking and labeling IP addresses. Hence, the CIDR notation system was created to standardize the process of labeling IP addresses. CIDR notation is a compact, streamlined method for representing IP addresses, their associated network or subnet mask, and the total number of bits in the address. It decreases the entries in the routing table and efficiently manages the address space. IP addresses following CIDR notation consist of a prefix and a suffix. The prefix refers to the binary representation of the network address, and the suffix represents the number of leading bits in the IP address, which is indicated using the “/” slash.

  Here’s a general representation of what CIDR notation looks like: 

  w.x.y.z/n

  Here, the suffix /n declares the total number of bits in the entire IP address.

  This is an example CIDR notation:

  214.100.48.00/20

  The suffix /20 indicates the number of leading bits in the binary form of the IP address.

  In other words, it represents a 20-bit network mask and can be used to indicate the network’s size without declaring the complete network mask in dotted decimals. Hence, the CIDR notation compactly represents the network mask for a complete binary address and the number of leading 1 bits. CIDR notation can be applied to both IPv4 and IPv6 addresses. While IPv4 addresses can contain a maximum of 32 bits, IPv6 allows for a maximum of 128 bits. Many CIDR calculators and other digital tools available in the market determine the IP range for a specific CIDR address quickly.
• What Are CIDR Blocks?

  What Are CIDR Blocks?

  A CIDR block is a contiguous block of IP addresses defined by a single CIDR (Classless Inter-Domain Routing) notation. It is the fundamental unit of address allocation and routing on the modern internet.

  The notation combines the starting IP address of the block with a prefix length (the number after the slash) specifying the size of the block.

  CIDR represents IP addresses and their packet routing traits through leading 1 bits and the prefix-based standard. CIDR blocks are collections of IP addresses with identical prefixes and number of bits. They have varying sizes—large CIDR blocks can be distinguished from smaller ones based on the prefix length. For instance, a shorter prefix means the block is bigger and can hold more IP addresses. Similarly, a larger prefix indicates the block is smaller and matches fewer IP addresses.

  IPv4 CIDR blocks can be identified using the following syntax: 

  a.b.c.d/n

  Here, the number after the slash represents the prefix length. When determining the network size, the dotted decimal portion is removed, and /n represents a CIDR block with an n-bit prefix.

  CIDR blocks lay the vital foundation for CIDR, since combining multiple CIDR blocks with a shared network prefix to form a more extensive routing network leads to supernetting. A supernet is defined as an element of a larger network containing adjacent CIDR blocks. Supernetting is also known as route aggregation or route summarization. It helps network administrators save address space and optimizes network routing decisions.

  CIDR Block Handling and Assignment

  The Internet Assigned Numbers Authority is responsible for managing and assigning CIDR blocks. The large blocks of IP addresses are issued to Regional Internet Registries (RIRs) handling single and larger geographic areas such as Europe, Africa, and North America. These RIRs then subdivide the large blocks into smaller subnets. The smaller blocks of IP addresses are further assigned to Local Internet Registries (LIRs). The subdividing process is continued until the blocks are finally assigned to end-user networks.

  The size of the blocks assigned differs depending on the address requirements of each end-user network. The IETF recommends end-user networks receiving blocks from a single internet service provider (ISP) obtain their IP address space via ISP. However, networks using multiple ISPs require provider-independent blocks from the appropriate LIR or RIR.
• How Does CIDR Work?

  How Does CIDR Work?

  CIDR is classless addressing and possesses a major advantage over the traditional classful addressing method, as it can declare prefixes of arbitrary lengths, making the process of IP address allocation more efficient. The older classful addressing method focused on dividing the address space into five classes: A, B, C, D, and E. However, only Classes A, B, and C are used for network hosts, which results in address exhaustion. This is because user requirements had to be rounded up to the next largest class. Classful addressing included the following:

  • Class A: Over 16 million IP addresses
  • Class B: 65,535 IP addresses
  • Class C: 254 IP addresses

  So if an end user required 400 IP addresses, they would have to go with Class B, where over 60,000 addresses would go to waste. Hence, the CIDR addressing scheme was introduced to tackle this challenge and has become an efficient method for IP address representation. Additionally, every ISP leverages CIDR as the routing system of choice.

  CIDR works on the principle of VLSM, where an IP address space is divided into subnets of varying sizes. A subnet refers to a network’s logical or physical subdivision containing a group of IP addresses with identical routing prefixes. Subnetting helps prevent network congestion by reducing the bandwidth load on the network.

  The concept of subnet masking leveraged by the CIDR addressing scheme involves splitting the IP address into the host and network addresses. A subnet mask is defined as a 32-bit number consisting of a sequence of number ones representing the network prefix, followed by a block of zeros representing the host identifier. Hence, routers are signaled by subnet masks about the part of the IP address assigned to the hosts and the part belonging to the network.

  Using CIDR, network engineers can also create supernets, where multiple continuous networks can be combined into an extensive network. Supernets enable you to integrate several selected routes into one general route advertisement. This is also called route aggregation, and data packets are routed only to one destination irrespective of the host location. Hence, one routing table entry for a supernet indicates a group of networks.

  Supernetting has many advantages, including reduced latency in complex networks with many routers and a minimized number of routing tables in an IP address. It is commonly used by organizations managing many hosts and networks. Routers following the CIDR approach use the destination address to route a data packet. When a router knows routes for different parts of the same supernet, it uses the one with the most extended network address.
• What Are the Advantages of Using CIDR Notation?

  What Are the Advantages of Using CIDR Notation?

  CIDR notation provides many advantages compared to the classful addressing scheme. Outlined below are some key benefits:

  • Optimized address space allocation: CIDR notation has significantly improved address allocation and reduced wasted IP addresses, because CIDR addresses are allocated in varying sizes of any binary multiple instead of the fixed-size blocks approach used in classful addressing; network identification is easier in CIDR, as multiple contiguous bits can be assigned based on the number of hosts requiring network support
  • Reduced routing table entries: CIDR notation has helped minimize the number of entries in a routing table, thereby improving the packet routing process (for instance, one routing table entry for a supernet in CIDR represents a group of networks); this capability to represent many networks through few routing entries is due to CIDR’s multiple-level hierarchical subnet structure—unlike classful addressing, where a separate entry is made in the routing table for each network
  • Straightforward designation method: The CIDR standard format represents the network mask as a single value, making it a more compact and concise method of subnet mask designation compared to dotted decimal notation, where the network mask is represented as four values; CIDR notation is also more suitable for multi-segment displays, where only three characters can be displayed at a time
• Disadvantages of CIDR

  Disadvantages of CIDR

  • Complexity: CIDR relies on VLSM, which requires network administrators to perform more complex binary calculations and detailed planning compared to the simple fixed-size classful system it replaced
  • Management overhead: CIDR block overlaps can easily occur in large evolving networks (such as those in cloud environments), leading to routing conflicts, so keeping track of subnets of various sizes (IP address management) is difficult
  • Fragmentation: Address space can become noncontiguous (fragmented) over time, which makes route aggregation (supernetting) less efficient, as continuous blocks are required for summarization, resulting in larger routing tables
  • Loss of granularity: When multiple specific routes are summarized into one larger CIDR block (aggregation), upstream routers lose detailed information about individual subnets, and this loss of detail can complicate troubleshooting and lead to routing black holes if a specific subnet within the aggregated block fails