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CIDR (Classless Inter-Domain Routing)

CIDR (Classless Inter-Domain Routing) is a method for allocating and routing IP addresses more efficiently.
It replaces the old classful addressing system, which was too rigid and caused waste of IP address space.

CIDR introduces the prefix length notation (e.g., /24) to define how many bits belong to the network portion of an address.

Table of Contents

  1. What is CIDR?
  2. Why Was CIDR Introduced?
  3. CIDR Notation
  4. CIDR Prefix and Host Capacity
  5. CIDR vs Classful Addressing
  6. CIDR in Routing
  7. CIDR and Subnetting
  8. Examples of CIDR
  9. Advantages of CIDR
  10. Further Reading

What is CIDR?

CIDR allows networks to be defined with any prefix length, not just fixed class boundaries (Class A, B, C).

  • Example: Instead of being forced to use a Class C (255.255.255.0 or /24), you can use /27 or /29 depending on your needs.

This flexibility avoids wasting addresses and enables more efficient routing.

Why Was CIDR Introduced?

Before CIDR, IP addresses were divided into classes:

  • Class A: 16 million hosts (wasted for small networks).
  • Class B: 65,534 hosts (still very large).
  • Class C: 254 hosts (often too small).

This rigid system caused:

  1. IP address exhaustion – huge blocks wasted.
  2. Routing table explosion – too many entries in global routers.

CIDR solved both problems by:

  • Allowing variable-length prefixes.
  • Supporting route aggregation (summarizing multiple routes into one).

CIDR Notation

CIDR uses slash notation to indicate the number of network bits.

Example:

  • 192.168.1.0/24 → 24 bits network, 8 bits host.
  • 192.168.1.0/26 → 26 bits network, 6 bits host.

CIDR Prefix and Host Capacity

CIDR Prefix Subnet Mask Network Bits Host Bits Usable Hosts
/8 255.0.0.0 8 24 16,777,214
/16 255.255.0.0 16 16 65,534
/24 255.255.255.0 24 8 254
/26 255.255.255.192 26 6 62
/30 255.255.255.252 30 2 2

Formula:

  • Usable hosts = (2^host bits) - 2

CIDR vs Classful Addressing

Classful addressing:

  • Fixed blocks: Class A (/8), Class B (/16), Class C (/24).
  • Wasteful: a small company could only get 65,000 addresses even if it needed 500.

CIDR addressing:

  • Flexible prefix length (e.g., /22, /28).
  • Networks sized according to need.
  • Aggregation reduces routing complexity.

CIDR in Routing

Routers use CIDR for route summarization (also called supernetting).

Example:

  • Instead of listing:
    • 192.168.0.0/24
    • 192.168.1.0/24
    • 192.168.2.0/24
    • 192.168.3.0/24

We can aggregate them as:

  • 192.168.0.0/22

This shrinks routing tables, improving efficiency and speed.

CIDR and Subnetting

  • Subnetting breaks a large block into smaller subnets.
  • CIDR generalizes this by allowing any prefix length.
  • CIDR is sometimes called “supernetting” when combining multiple networks.

So, subnetting and CIDR are two sides of the same coin:

  • Subnetting = dividing networks.
  • CIDR = combining or flexibly defining them.

Examples of CIDR

  1. Small Office Network

    • Need ~50 hosts.
    • /26 gives 62 usable hosts → perfect.

  2. ISP Aggregation

    • ISP owns 200.10.0.0 – 200.10.15.255.
    • Instead of advertising 16 Class C routes, ISP announces:
      • 200.10.0.0/20

Advantages of CIDR

  • Efficient use of IPv4 space.
  • Delayed IPv4 exhaustion.
  • Smaller routing tables.
  • Flexibility: networks can be exactly as large as needed.
  • Foundation for modern IP routing and internet backbone.

Further Reading