What Is CIDR Notation? The Complete Guide with Calculations and Subnetting Chart

1. The Evolution: Why CIDR Replaced Classful Networking

Prior to 1993, the global internet allocated addresses in rigid, fixed buckets known as Classful Addressing:

- Class A (/8): 16,777,214 hosts per network. Only 126 Class A networks existed globally, allocated to multinational giants like IBM, Apple, and MIT.

- Class B (/16): 65,534 hosts per network. Exhausted almost immediately by universities and mid-sized companies.

- Class C (/24): 254 hosts per network. Too small for growing organizations, forcing companies to request multiple /24 blocks.

This caused rapid IP exhaustion and explosive growth in global BGP routing tables. The IETF deployed CIDR (RFC 1519 and RFC 4632), introducing variable-length prefix allocation and route aggregation (supernetting).

2. How Slash Notation Works: Binary Bitmask Mechanics

In CIDR notation, an IP address is followed by a forward slash and an integer (such as 10.0.0.0/24). The integer represents the prefix length — the exact count of leading contiguous binary 1s in the 32-bit subnet mask.

The remaining bits represent host capacity. The usable host formula is strictly: 2^(32 - Prefix) - 2.

Example CIDR Block:   192.168.1.0/26
Prefix Length:        26 bits
Subnet Mask:          255.255.255.192
Binary Subnet Mask:   11111111.11111111.11111111.11000000
Host Bits Remaining:  32 - 26 = 6 bits
Total Addresses:      2^6 = 64 addresses
Usable Host Capacity: 64 - 2 = 62 usable endpoints

3. Complete CIDR Prefix Reference Table (/8 to /32)

This engineering table details all standard CIDR prefixes, their corresponding dotted decimal masks, total IP counts, and typical deployment scopes:

4. CIDR Notation in Cloud Virtual Private Clouds (AWS, GCP, Azure)

When designing cloud architectures in AWS VPC, Google Cloud VPC, or Microsoft Azure VNet, CIDR notation defines the network perimeter:

1. VPC Root CIDR: Typically sized as a /16 block (e.g., 10.0.0.0/16) providing 65,534 IP addresses.

2. Subnet Tiers: Partitioned into /24 subnets across Availability Zones (e.g., Public Web Tier: 10.0.1.0/24; Private Application Tier: 10.0.2.0/24; Database Tier: 10.0.3.0/24).

Crucial Cloud Note: Cloud providers reserve additional IPs! In AWS, each subnet reserves 5 addresses (Network, VPC Router .1, DNS .2, Future Use .3, and Broadcast .255). A /24 in AWS has 251 usable instances, not 254.