CIDR to Netmask Converter API: The Complete Guide

Need to convert CIDR prefix notation to traditional subnet masks? This guide covers everything about CIDR to netmask conversion via API, including the conversion math, common values, and implementation examples.

Why Convert CIDR to Netmask?

While CIDR notation is the modern standard, many legacy systems, network equipment, and older configurations still require subnet masks in dotted-decimal format.

Converting /24 to 255.255.255.0 ensures compatibility with these systems while working with modern CIDR-based tools.

The Conversion Process

The netmask is created by setting the first N bits to 1, where N is the CIDR prefix:

/24 means 24 bits of 1s followed by 8 bits of 0s:
11111111.11111111.11111111.00000000

Convert each octet to decimal:
255.255.255.0

Using the CIDR to Netmask API

TinyFn provides a simple endpoint for CIDR conversion:

GET https://api.tinyfn.io/v1/ip/cidr-to-netmask?cidr=24
Headers: X-API-Key: your-api-key

{
  "cidr": 24,
  "netmask": "255.255.255.0",
  "binary": "11111111111111111111111100000000",
  "wildcard": "0.0.0.255",
  "total_addresses": 256,
  "usable_hosts": 254
}

Parameters

Code Examples

JavaScript / Node.js

const response = await fetch(
  'https://api.tinyfn.io/v1/ip/cidr-to-netmask?cidr=24',
  { headers: { 'X-API-Key': 'your-api-key' } }
);
const { netmask } = await response.json();
console.log(`Netmask: ${netmask}`); // "Netmask: 255.255.255.0"

Python

import requests

response = requests.get(
    'https://api.tinyfn.io/v1/ip/cidr-to-netmask',
    headers={'X-API-Key': 'your-api-key'},
    params={'cidr': 24}
)
netmask = response.json()['netmask']
print(f"Netmask: {netmask}")  # "Netmask: 255.255.255.0"

cURL

curl "https://api.tinyfn.io/v1/ip/cidr-to-netmask?cidr=24" \
  -H "X-API-Key: your-api-key"

Common Use Cases

• Legacy System Configuration: Configure older routers and firewalls
• Network Scripts: Generate configs for various equipment types
• Documentation: Include both formats in network documentation
• Training Materials: Show both representations for learning
• API Translation: Bridge between CIDR and netmask-based APIs

Best Practices

1. Validate prefix: Ensure CIDR prefix is between 0 and 32
2. Include wildcard: Some systems need wildcard mask instead of netmask
3. Document both formats: Show CIDR and netmask in documentation
4. Test configurations: Verify converted values work in target systems

Use via MCP

Your AI agent can call this tool directly via Model Context Protocol — no HTTP code needed. Add TinyFn to Claude Desktop, Cursor, or any MCP client:

{
  "mcpServers": {
    "tinyfn-ip": {
      "url": "https://api.tinyfn.io/mcp/ip/",
      "headers": {
        "X-API-Key": "your-api-key"
      }
    }
  }
}

See all ip address tools available via MCP in our IP Address MCP Tools for AI Agents guide.