CIDR in Networking

What is CIDR in Networking? Complete Overview

Concept: Classless Inter-Domain Routing (CIDR) | Introduced: Early 1990s | Goal: Efficient IP allocation & scalable routing

CIDR, or Classless Inter-Domain Routing, is a modern IP addressing scheme that replaces the old Class A, B, and C–based system to make much better use of IPv4 address space and shrink global routing tables. [web:23][web:25] By using variable-length prefixes like /24 or /20, CIDR allows flexible subnetting and route summarization instead of rigid classful blocks. Read more about CIDR on AWS .

Key CIDR Features and Concepts

• Classless addressing: CIDR removes fixed Class A, B, C boundaries and uses a prefix length (for example /24) to define the network portion. [web:23][web:26]
• CIDR notation: An IP address is written with a suffix like 192.168.1.0/24, where /24 indicates how many bits form the network prefix. [web:27][web:30]
• Variable-Length Subnet Masking (VLSM): CIDR enables networks to be split into different-sized subnets, closely matching host requirements and reducing address waste. [web:26][web:33]
• Route summarization (supernetting): Multiple contiguous networks can be aggregated into a single prefix, which keeps routing tables smaller and more efficient. [web:22][web:28]
• IPv4 exhaustion mitigation: By allocating blocks that fit actual needs, CIDR slowed IPv4 address depletion compared with the older classful scheme. [web:25][web:26]
• Applicability to IPv6: CIDR-style prefix notation is also used with IPv6 to define network and subnet boundaries. [web:23][web:35]
• Fine‑grained control: Network engineers can design address plans with /30, /28, /20, or other custom sizes instead of being locked to 256‑address Class C blocks. [web:27][web:30]
• Improved scalability: Internet backbone routers handle fewer, aggregated routes, improving performance and scalability as the network grows. [web:23][web:28]

CIDR Notation Examples

Basic CIDR Notation

# 192.168.1.0/24
# Network: 192.168.1.0
# Prefix length: 24 bits for network
# Host bits: 8 (2^8 = 256 total addresses)

Small Subnet for Point-to-Point Links

# 10.0.0.0/30
# Subnet mask: 255.255.255.252
# Total addresses: 4 (2 usable for hosts on many systems)

Creating a Larger Supernet

# Aggregating two /24s into one /23
# 192.168.0.0/24 and 192.168.1.0/24
# can be summarized as 192.168.0.0/23

CIDR vs Classful Allocation

# Classful:
# Company with 100 hosts might receive a Class C block (256 addresses)
# > 150 addresses wasted.

# CIDR:
# 192.0.2.0/25
# 128 addresses, closer to actual requirement, far less waste.

CIDR Feature Summary Table

Concept	Description
CIDR Definition	Method of assigning IP addresses and routing prefixes without fixed classes, improving address utilization and routing efficiency. [web:25][web:26]
CIDR Notation	IP address plus a slash and prefix length (for example 203.0.113.0/24) indicating network bits. [web:27][web:30]
VLSM	Variable-Length Subnet Masking splits address blocks into differently sized subnets to match host counts. [web:26][web:33]
Route Summarization	Combines multiple contiguous networks into one aggregated route, shrinking routing tables. [web:22][web:28]
IPv4 Conservation	Allocates only the number of addresses needed, slowing IPv4 address exhaustion. [web:25][web:34]
IPv6 Prefixing	Uses the same prefix-length idea (for example /64) to define IPv6 network boundaries. [web:23][web:35]
Scalability Benefit	Makes global routing more scalable by reducing the number of advertised routes through aggregation. [web:23][web:28]

What Exactly is CIDR?

CIDR (Classless Inter-Domain Routing) is a set of IP addressing standards that use a flexible prefix length rather than rigid address classes, which optimizes address allocation and routing. [web:23][web:26] In practice, CIDR lets network designers describe the network portion by how many bits are used, written as a “slash” value after the IP address. [web:27][web:30]

• Prefix length: The number after the slash (for example /24) tells how many bits belong to the network portion. [web:30][web:33]
• Flexible subnet sizes: Networks can be divided into /25, /26, /28, and many other sizes, instead of a single default mask per class. [web:26][web:27]
• Supernetting: Multiple adjacent networks can be grouped into one larger block like 192.168.0.0/23 to reduce routing entries. [web:22][web:28]
• Decoupled from classes: Allocation is no longer bound to Class A, B, or C ranges; blocks are given strictly according to need. [web:23][web:25]
• Better utilization: Addresses that would be unused in classful blocks can be reclaimed and assigned elsewhere. [web:26][web:34]

Example: Classful vs CIDR Allocation

# Classful addressing example
# Small company with ~100 devices
# Might receive a full Class C block:
# 198.51.100.0 with mask 255.255.255.0
# 256 total addresses, ~156 wasted.

# CIDR addressing example
# Same company receives 198.51.100.0/25
# Mask 255.255.255.128
# 128 total addresses, much closer to 100 devices and far less waste.

Want to master IP subnetting and CIDR planning? Explore this detailed IP addressing and CIDR guide for more real‑world examples and practice scenarios.

CIDR was designed to slow IPv4 address exhaustion and reduce the growth of global routing tables by replacing large, fixed classful blocks with flexible prefixes. [web:25][web:26]

✅ Example of waste with classful addressing

# Class C block:
# 203.0.113.0 with mask 255.255.255.0
# Total addresses: 256
# Company needs only 40 hosts → > 200 addresses unused.
          

In the classful model, organizations often received far more addresses than they needed, leading to severe under‑utilization of IPv4 space. [web:28][web:34]

✅ Example of efficient CIDR allocation

# CIDR block:
# 203.0.113.0/26
# Mask: 255.255.255.192
# Total addresses: 64
# Much closer to the 40‑host requirement.
          

By using CIDR prefixes, the remaining address space can be allocated to other networks instead of being locked inside a single oversized block. [web:26][web:27]

🔁 Key Benefits

• Address conservation: Reduces wastage and extends the lifetime of IPv4 space. [web:25][web:34]
• Smaller routing tables: Route summarization allows multiple networks to appear as one entry. [web:22][web:28]

Tip: When designing an address plan, always start from host requirements and choose the smallest CIDR prefix that covers them. [web:27][web:30]

CIDR notation expresses an IP address followed by a slash and a prefix length, indicating how many bits belong to the network portion. [web:27][web:30]

✅ Example: 192.168.10.0/24

IP address: 192.168.10.0
Prefix length: /24  →  24 bits network, 8 bits host
Subnet mask: 255.255.255.0
Total addresses: 256 (254 typically usable)
          

The higher the prefix length (for example /28, /30), the fewer host addresses remain available in the subnet. [web:27][web:30]

🔒 Benefits of CIDR notation

• Shorter and clearer than writing full subnet masks. [web:33]
• Works consistently for IPv4 and IPv6 networks. [web:23][web:35]
• Makes it easier to reason about subnet sizes and address planning. [web:27][web:30]

Note: The CIDR number equals the count of consecutive 1 bits in the subnet mask when written in binary (for example 255.255.255.0 has 24 ones → /24). [web:30]

Classful addressing used fixed ranges (Class A, B, C) with default masks, while CIDR ignores classes and uses variable prefix lengths for any range. [web:23][web:26]

• Classful networks often gave organizations far more addresses than they needed. [web:28][web:34]
• CIDR enables precise, variable‑sized allocations that match real demand. [web:26][web:27]
• CIDR supports route aggregation, whereas classful addressing led to more, smaller routes. [web:22][web:28]

✅ Example comparison

Classful: 10.0.0.0/8 for a large org → 16 million addresses.
CIDR:     10.0.0.0/16, /20, or /24 subnets assigned per site as needed.
          

This flexibility is why modern ISPs and enterprises rely on CIDR‑based allocations instead of classful blocks. [web:28][web:34]

Note: You may still see “Class A/B/C” in learning material, but real networks are designed and routed using CIDR prefixes. [web:23][web:26]

CIDR allows ISPs to advertise one aggregated prefix that covers many customer networks, greatly reducing the number of routes core routers must maintain. [web:22][web:28]

✅ Example: Route aggregation

Customer blocks:
203.0.113.0/25
203.0.113.128/25

ISP can advertise:
203.0.113.0/24
          

• 203.0.113.0/24 summarizes multiple smaller subnets into one entry. [web:22][web:28]
• Fewer routes mean faster convergence and lower memory usage on routers. [web:23][web:28]
• Hierarchical, CIDR‑based design keeps the global routing system scalable. [web:23][web:34]

Key point: CIDR’s ability to aggregate prefixes is as important as its address‑saving benefits when designing large‑scale networks. [web:22][web:28]

CIDR planning starts from how many hosts you need, then selects the smallest prefix length that provides enough addresses with some growth room. [web:27][web:30]

• For ~50 hosts, /26 (64 addresses) is often a good fit. [web:30]
• For point‑to‑point links, /30 or even /31 may be appropriate on supported systems. [web:30]
• For large LANs, prefixes like /23 or /22 can be used instead of multiple /24s. [web:27][web:30]

Many subnet calculators and cloud VPC wizards will suggest suitable CIDR blocks once you specify host counts or number of subnets. [web:22][web:27]

✅ Quick /24 breakdown example

Start with 192.0.2.0/24 (256 addresses)
Split into:
- 192.0.2.0/25   → 128 addresses
- 192.0.2.128/25 → 128 addresses
          

Note: Always reserve some extra space for future hosts or additional subnets when designing CIDR blocks. [web:22][web:27]

Frequently Asked Questions

Q1. What is CIDR in simple terms?
A: CIDR is a way of writing and allocating IP addresses using flexible prefix lengths, so networks get only as many addresses as they actually need. [web:25][web:26]

Q2. Why is CIDR still important today?
A: CIDR remains essential for conserving IPv4 space, designing scalable IPv6 networks, and keeping Internet routing tables small through route aggregation. [web:23][web:28][web:35]