CIDR Calculator Network Tool

Enter an IP address and CIDR prefix to calculate precise network details, subnet masks, and address ranges for efficient network planning.

Documentation

What is CIDR?

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing Internet Protocol packets. It replaces the older class-based system with a more flexible approach using variable-length subnet masking (VLSM).

A CIDR notation consists of an IP address followed by a slash and a number (e.g., 192.168.1.0/24). The number after the slash indicates how many bits are used for the network portion of the address.

How It Works

  1. Input Validation: Validates IPv4 address format
  2. Binary Conversion: Converts IP to 32-bit binary
  3. Network Calculation: Determines network and broadcast addresses
  4. Host Range: Calculates usable host addresses
  5. Capacity: Computes total and usable host counts

Common Use Cases

  • Network planning and subnetting
  • VPN and remote access configuration
  • Cloud networking (AWS, Azure, GCP)
  • Firewall and security group rules
  • Network troubleshooting

Here are some common CIDR examples with their calculated network details:

192.168.1.0/24

Small Network
Network: 192.168.1.0
Broadcast: 192.168.1.255
Subnet Mask: 255.255.255.0
Usable Hosts: 254

10.0.0.0/8

Large Network
Network: 10.0.0.0
Broadcast: 10.255.255.255
Subnet Mask: 255.0.0.0
Usable Hosts: 16,777,214

172.16.0.0/16

Medium Network
Network: 172.16.0.0
Broadcast: 172.16.255.255
Subnet Mask: 255.255.0.0
Usable Hosts: 65,534

192.168.1.1/32

Host Route
Network: 192.168.1.1
Broadcast: 192.168.1.1
Subnet Mask: 255.255.255.255
Usable Hosts: 1

This CIDR Calculator is implemented using pure JavaScript with no external dependencies. The calculation logic is based on binary manipulation of IP addresses for maximum accuracy.

Step-by-Step Code Breakdown

1. IP Address Validation

First, we validate that the input is a proper IPv4 address:

const ipParts = ip.split('.').map(Number);
// Split IP string into octets and convert to numbers
if (ipParts.length !== 4 || ipParts.some(p => p < 0 || p > 255)) {
    return '<p>Invalid IP address.</p>';
}
// Ensure exactly 4 octets, each between 0-255

2. Binary Conversion

Convert the IP address to a 32-bit binary string for bitwise operations:

const ipBinary = ipParts.map(p => p.toString(2).padStart(8, '0')).join('');
// Convert each octet to 8-bit binary and concatenate

This creates a 32-bit string representation of the IP address.

3. Network and Broadcast Calculation

The network address is the first address in the subnet, with all host bits set to 0:

const networkBinary = ipBinary.substring(0, cidr) + '0'.repeat(32 - cidr);
// Keep first 'cidr' bits, set remaining bits to 0

The broadcast address is the last address in the subnet, with all host bits set to 1:

const broadcastBinary = ipBinary.substring(0, cidr) + '1'.repeat(32 - cidr);
// Keep first 'cidr' bits, set remaining bits to 1

4. IP Address Reconstruction

Convert binary strings back to dotted decimal format:

function binaryToIP(binary) {
    return [
        parseInt(binary.substring(0, 8), 2),   // First octet (bits 0-7)
        parseInt(binary.substring(8, 16), 2),  // Second octet (bits 8-15)
        parseInt(binary.substring(16, 24), 2), // Third octet (bits 16-23)
        parseInt(binary.substring(24, 32), 2)  // Fourth octet (bits 24-31)
    ].join('.');
}

This helper function converts a 32-bit binary string back to IPv4 dotted decimal notation.

5. Host Address Calculation

Calculate the first usable host address (network address + 1):

const firstHost = cidr < 31 ? binaryToIP(networkBinary.substring(0, 31) + '1') : networkIP;
// For /31 and /32, network address is usable

Calculate the last usable host address (broadcast address - 1):

const lastHost = cidr < 31 ? binaryToIP(broadcastBinary.substring(0, 31) + '0') : broadcastIP;
// For /31 and /32, broadcast address is usable

6. Host Count Calculation

Total addresses in the subnet (including network and broadcast):

const totalHosts = cidr <= 30 ? Math.pow(2, 32 - cidr) : 1;
// 2^(32-cidr) for /0 to /30, 1 for /31 and /32

Usable addresses for host assignment (excluding network and broadcast):

const usableHosts = cidr <= 30 ? totalHosts - 2 : 1;
// Total minus 2 for /0 to /30, 1 for /31 and /32

Edge Cases and Special Considerations

/31 Prefixes (RFC 3021)

Point-to-point links with 2 total hosts, both usable (no network/broadcast addresses).

/32 Prefixes

Host routes with 1 total and usable host (single IP address).

/0 Prefix

Represents the entire IPv4 address space (0.0.0.0/0).

Binary Precision

All calculations use 32-bit binary arithmetic for accuracy and consistency.

Subnetting Tips

  • Choose appropriate prefix lengths: /24 for small networks, /16 for larger subnets, /8 for very large networks.
  • Plan for growth: Allocate more addresses than currently needed to avoid renumbering later.
  • Use VLSM: Variable Length Subnet Masks allow efficient use of address space by creating subnets of different sizes.
  • Document your subnets: Keep track of allocated subnets to avoid conflicts and aid troubleshooting.
  • Consider RFC 1918: Use private address ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) for internal networks.

Network Planning

  • Departmental subnets: Use /24 for departments with 250+ devices
  • VLAN design: Plan subnets per VLAN for security and management
  • Future expansion: Leave room for growth in subnet allocations
  • Address conservation: Use the smallest subnet that meets current needs
  • Hierarchical design: Organize subnets in a logical hierarchy

Security Considerations

  • Firewall rules: Use CIDR notation for precise access control
  • Network segmentation: Isolate sensitive systems in separate subnets
  • Access control: Implement least privilege with subnet restrictions
  • Monitoring: Log and monitor traffic between subnets
  • Compliance: Ensure subnet design meets security requirements