Examining Classless Inter-Domain Routing (CIDR)

What is Classless Inter-Domain Routing (CIDR)?

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing Internet Protocol packets. The main idea behind CIDR is to allocate blocks of IP addresses to organizations based on their needs, while trying to conserve the remaining IP address space.

Some key points about CIDR:

  • It was introduced in 1993 as a replacement for the original system of IP address classes (Class A, B, C, etc). The class system wasted too much address space.
  • CIDR allows for variable-length subnet masking (VLSM), so organizations can be allocated any arbitrary block of addresses. For example, a /24 would be 256 IP addresses.
  • IP addresses are assigned in blocks based on network prefixes, written in slash notation. For example, represents the 1024 IP addresses from to
  • The remaining (host) bits in the prefix can be assigned flexibly to local networks. This allows efficient aggregation of routing information for routing scalability.
  • CIDR and VLSM help slow the rate of IPv4 address exhaustion by better optimizing allocation. However, IPv6 has vastly more addresses and makes some of these conservation tactics less necessary.

CIDR provides an efficient and flexible way to allocate blocks of IP addresses and scale routing on the Internet. It replaced the older class system to make better use of limited IPv4 space.

How is IP address calculated in CIDR?

Here is how IP addresses are calculated and represented in CIDR notation:

  • The CIDR notation looks like -
  • The /24 part represents the prefix length. This means the first 24 bits are the network prefix.
  • Convert the prefix length to a binary netmask:
    /24 =
  • Apply this netmask to the IP address to separate the network and host portions: = Network address = Netmask

  • So & gives us the network address
  • The remaining 0 bits in the netmask mean those bits can be used for host addresses.
  • So in a /24, the last 8 bits (32 - 24 = 8) can be used for 256 host addresses.
  • Therefore, the range of usable hosts in a /24 is to
  • The network address and broadcast address are not usable.

The prefix length defines the netmask, which separates the network and host portions of the IP address. This allows calculating the range of usable IP addresses in a subnet. CIDR notation provides a compact way to represent networks and subnets of addresses.

What is the major advantage of CIDR?

Here are some of the major advantages of CIDR (Classless Inter-Domain Routing):

  • Efficient allocation of IP addresses: By allowing flexible subnet sizes, CIDR allows IP address space to be allocated more efficiently and appropriately based on an organization's actual needs.
  • Conservation of IP space: CIDR helps slow IPv4 address exhaustion by enabling better aggregation and allocation. This was crucial in the 1990s when address space was running low.
  • Route aggregation: CIDR allows routers to store fewer routes by aggregating multiple networks into larger routing prefixes. This improves scalability and reduces routing table size.
  • Simplification: CIDR got rid of the complex classful network scheme. Everything is now based on network prefixes instead of address classes.
  • Flexibility: Organizations can be allocated any prefix length to meet their subnetting needs rather than fixed sizes based on classes.
  • Migration path: CIDR provided a transition from classful routing to the classless approach. This made adoption feasible.
  • Address fragmentation: CIDR reduces fragmentation of address space into many small unusable chunks. This promotes better utilization.

Overall, the advantages of CIDR stem from its use of variable length subnets, route aggregation, simplified allocation, and better IP space utilization - critical improvements over the old classful system.

Can two subnets have same CIDR?

No, two different subnets cannot have the same CIDR or network prefix. The CIDR notation encodes the network address and subnet mask, which together uniquely identify a network segment.

Some key reasons two subnets cannot have identical CIDR notations:

  • The network prefix portion of the CIDR contains the unique network address for that segment. For example, and are different /24 subnets.
  • The prefix length (/24) also is part of identifying the subnet. Two networks with the same addresses but different prefixes are different subnets.
  • If two subnets had the same CIDR, routers would not have a consistent way to distinguish traffic bound for each network.
  • Overlapping subnets would lead to addressing conflicts, ambiguity, and routing issues.
  • Administrative control and delegation boundaries require subnets to have unique addresses and prefixes.
  • The entire point of CIDR is to provide a shorthand for unambiguously defining networks and dividing IP space efficiently.

The CIDR notation is designed to uniquely identify subnets to eliminate addressing conflicts and routing problems. The network address and prefix length must both be unique in order for two subnets to be considered different.