DevToolkit Pro

What Is CIDR

CIDR (Classless Inter-Domain Routing) is a flexible method for allocating IP addresses and routing. It replaced the traditional classful addressing system (Class A, B, C), allowing more granular address space allocation.

CIDR notation appends a prefix length after the IP address with a slash:

192.168.1.0/24
             ^^
             Prefix length (network bits)

Why You Need a CIDR Calculator

1. VPC network planning: Subnet design on cloud providers (AWS, Alibaba Cloud)
2. Container networks: Kubernetes Pod network configuration
3. Firewall rules: IP allowlist CIDR range configuration
4. Server deployment: Internal IP address planning
5. CDN configuration: IP range restrictions and edge node planning
6. Security auditing: Analyzing IP range coverage

CIDR Fundamentals

IP Address Structure

An IPv4 address is a 32-bit binary number, typically shown in dotted decimal:

192.168.1.100
= 11000000.10101000.00000001.01100100

First 24 bits = network (/24), last 8 bits = hosts

Subnet Mask

The subnet mask distinguishes network from host portions:

| CIDR Prefix | Subnet Mask | Usable Hosts | Use Case | |-------------|-------------|-------------|----------| | /8 | 255.0.0.0 | 16,777,214 | Large organizations | | /16 | 255.255.0.0 | 65,534 | Medium networks | | /24 | 255.255.255.0 | 254 | Small networks (most common) | | /28 | 255.255.255.240 | 14 | Cloud VPC subnets | | /32 | 255.255.255.255 | 1 | Single host |

Usable Host Formula

Usable hosts = 2^(32 - prefix_length) - 2

Minus 2 because:

• Network address (all host bits 0) is not assignable
• Broadcast address (all host bits 1) is not assignable

Common CIDR Blocks

| CIDR | IPs | Description | |------|-----|-------------| | /32 | 1 | Single host | | /31 | 2 | Point-to-point link | | /30 | 4 | WAN link (2 usable IPs) | | /29 | 8 | Small subnet | | /28 | 16 | Common in cloud VPCs | | /27 | 32 | Medium subnet | | /24 | 256 | Class C size | | /16 | 65,536 | Class B size |

How to Use an Online CIDR Calculator

Using DevToolkit Pro's CIDR Calculator:

1. Enter an IP address and CIDR prefix (e.g., 192.168.1.0/24)
2. The tool automatically calculates:
   • Network address
   • Broadcast address
   • Subnet mask
   • Usable IP range
   • Host count
3. Supports IP range to CIDR conversion

Cloud VPC Network Planning Example

Alibaba Cloud VPC Subnet Design

VPC CIDR: 10.0.0.0/16 (65,536 IPs)

├── Public subnet:     10.0.1.0/24  (254 IPs) ← Web servers
├── App subnet:        10.0.2.0/24  (254 IPs) ← App servers
├── Database subnet:   10.0.3.0/24  (254 IPs) ← Databases
├── Management subnet: 10.0.4.0/28  (14 IPs)  ← Bastion hosts
└── Kubernetes:        10.0.5.0/24  (254 IPs) ← Pod network

FAQ

What's the Difference Between /32 and /31?

/32 is a host route (exact match for one machine). /31 is a special format for point-to-point links that eliminates the need for network and broadcast addresses — both IPs are assignable to endpoints.

What Are Subnet Design Best Practices?

1. Segment by function (web, app, database)
2. Segment by environment (dev, staging, production)
3. Reserve 20%+ for future expansion
4. Use private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)

How Do I Check If Two CIDRs Overlap?

If two CIDRs produce the same result when ANDed with each other's subnet mask, they overlap. Online CIDR calculators can detect overlap directly.

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