Cidr Subnet Calculator

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CIDR Subnet Calculator :root { –primary-blue: #004a99; –success-green: #28a745; –light-background: #f8f9fa; –border-color: #dee2e6; –text-dark: #343a40; –text-muted: #6c757d; } body { font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif; background-color: var(–light-background); color: var(–text-dark); line-height: 1.6; margin: 0; padding: 20px; } .loan-calc-container { max-width: 800px; margin: 30px auto; background-color: #ffffff; padding: 30px; border-radius: 8px; box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1); } h1, h2 { color: var(–primary-blue); text-align: center; margin-bottom: 20px; } .input-group { margin-bottom: 20px; display: flex; flex-direction: column; gap: 8px; } .input-group label { font-weight: bold; color: var(–text-muted); display: block; } .input-group input[type="text"], .input-group input[type="number"] { width: 100%; padding: 10px 12px; border: 1px solid var(–border-color); border-radius: 4px; box-sizing: border-box; font-size: 1rem; } .input-group input:focus { border-color: var(–primary-blue); outline: none; box-shadow: 0 0 0 3px rgba(0, 74, 153, 0.2); } button { background-color: var(–primary-blue); color: white; border: none; padding: 12px 20px; border-radius: 4px; cursor: pointer; font-size: 1.1rem; transition: background-color 0.3s ease; width: 100%; margin-top: 10px; } button:hover { background-color: #003366; } #result { margin-top: 30px; padding: 20px; background-color: var(–primary-blue); color: white; border-radius: 8px; text-align: center; font-size: 1.5rem; font-weight: bold; box-shadow: 0 2px 10px rgba(40, 167, 69, 0.3); } #result span { font-size: 1.1rem; font-weight: normal; color: rgba(255, 255, 255, 0.9); } #result div { margin-top: 10px; font-size: 1.2rem; } .article-content { margin-top: 40px; background-color: #ffffff; padding: 30px; border-radius: 8px; box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1); } .article-content h2 { text-align: left; color: var(–primary-blue); border-bottom: 2px solid var(–border-color); padding-bottom: 10px; margin-bottom: 20px; } .article-content p, .article-content ul, .article-content li { margin-bottom: 15px; color: var(–text-muted); } .article-content code { background-color: var(–light-background); padding: 2px 6px; border-radius: 3px; font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace; } .article-content strong { color: var(–text-dark); } /* Responsive Adjustments */ @media (max-width: 600px) { .loan-calc-container { padding: 20px; } h1 { font-size: 1.8rem; } #result { font-size: 1.3rem; } button { font-size: 1rem; } }

CIDR Subnet Calculator

Enter IP Address and CIDR Mask to calculate.

Understanding CIDR and Subnetting

The CIDR (Classless Inter-Domain Routing) subnet calculator is an essential tool for network administrators and IT professionals. It helps in determining the network address, broadcast address, and the number of usable IP addresses within a given subnet, based on an IP address and its CIDR notation.

What is CIDR?

CIDR, introduced in 1993, revolutionized IP address allocation by moving away from the traditional classful addressing system (Class A, B, C). It allows for more flexible and efficient allocation of IP addresses by using a prefix length to define the network portion of an IP address. The notation consists of an IP address followed by a forward slash and a number representing the number of bits used for the network portion. For example, 192.168.1.0/24 means the first 24 bits of the IP address represent the network, and the remaining bits represent the host.

How Subnetting Works

Subnetting is the process of dividing a larger IP network into smaller subnetworks. This is achieved by borrowing bits from the host portion of an IP address to create a subnet portion. The CIDR prefix length directly indicates how many bits are used for the network and subnet combined.

  • Network Address: This is the first IP address in a subnet and is used to identify the subnet itself. All host bits are set to 0.
  • Broadcast Address: This is the last IP address in a subnet and is used to send data to all hosts within that subnet. All host bits are set to 1.
  • Usable IP Addresses: These are the IP addresses between the network address and the broadcast address that can be assigned to devices (hosts) on the network. The number of usable IPs is calculated as 2(32 – CIDR Prefix) – 2. The '-2' accounts for the network and broadcast addresses, which cannot be assigned to hosts.
  • Subnet Mask: This is derived from the CIDR prefix. A /24 CIDR prefix, for example, corresponds to a subnet mask of 255.255.255.0. It's a 32-bit number where the first 'n' bits (where 'n' is the CIDR prefix) are set to 1, and the remaining bits are set to 0.

The Math Behind the Calculator

  1. IP Address to Binary: The input IP address (e.g., 192.168.1.0) is converted into its 32-bit binary representation.
    Example: 192.168.1.0 becomes 11000000.10101000.00000001.00000000
  2. CIDR Prefix to Mask: The CIDR prefix (e.g., 24) determines the subnet mask. The first 'n' bits are 1s, and the rest are 0s.
    Example: /24 becomes 24 ones followed by 8 zeros: 11111111.11111111.11111111.00000000, which is 255.255.255.0.
  3. Network Address Calculation: The binary representation of the IP address is ANDed with the binary representation of the subnet mask.
    Example: 
    11000000.10101000.00000001.00000000  (IP)
AND 11111111.11111111.11111111.00000000  (Mask)
------------------------------------
= 11000000.10101000.00000001.00000000  (Network Address)
    This binary result is converted back to dotted-decimal notation: 192.168.1.0.
  4. Broadcast Address Calculation: To find the broadcast address, take the binary network address and set all the host bits (bits after the CIDR prefix) to 1.
    Example: For /24, the last 8 bits are host bits. 
    11000000.10101000.00000001.00000000  (Network Address)
OR  00000000.00000000.00000000.11111111  (Inverted Mask / Host Bits)
------------------------------------
= 11000000.10101000.00000001.11111111  (Broadcast Address)
    This binary result is converted back to dotted-decimal notation: 192.168.1.255.
  5. Usable IP Addresses: The total number of addresses in the subnet is 2(32 – CIDR Prefix). Subtract 2 for the network and broadcast addresses to get the usable count.
    Example: For /24, 32 – 24 = 8 host bits. 28 = 256. Usable IPs = 256 – 2 = 254.

Use Cases

This calculator is invaluable for:

  • Network Design: Planning efficient IP address allocation for new networks or expanding existing ones.
  • Troubleshooting: Identifying network boundaries and potential IP conflicts.
  • Security: Defining specific network segments for access control and policy enforcement.
  • Education: Learning and understanding the fundamentals of IP addressing and subnetting.
function calculateSubnet() { var ipAddress = document.getElementById("ipAddress").value; var cidrMask = parseInt(document.getElementById("cidrMask").value); var resultDiv = document.getElementById("result"); // Reset previous results and styles resultDiv.innerHTML = 'Enter IP Address and CIDR Mask to calculate.'; resultDiv.style.backgroundColor = 'var(–primary-blue)'; // Default back to primary blue // — Input Validation — if (!ipAddress || !/^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$/.test(ipAddress)) { resultDiv.innerHTML = 'Invalid IP Address format.'; return; } var octets = ipAddress.split('.').map(Number); if (octets.some(octet => octet 255)) { resultDiv.innerHTML = 'IP Address octets must be between 0 and 255.'; return; } if (isNaN(cidrMask) || cidrMask 32) { resultDiv.innerHTML = 'CIDR Mask must be between 0 and 32.'; return; } // — Calculations — var ipBinary = octets.map(octet => octet.toString(2).padStart(8, '0')).join("); var maskBinary = '1'.repeat(cidrMask) + '0'.repeat(32 – cidrMask); var networkBinary = "; var broadcastBinary = "; for (var i = 0; i < 32; i++) { var ipBit = ipBinary[i]; var maskBit = maskBinary[i]; var maskBitInverted = maskBinary[i] === '0' ? '1' : '0'; // Network Address: IP AND Mask networkBinary += (ipBit === '1' && maskBit === '1') ? '1' : '0'; // Broadcast Address: Network OR Inverted Mask bits for host part if (i < cidrMask) { broadcastBinary += networkBinary[i]; // Use the determined network bits } else { broadcastBinary += '1'; // Set host bits to 1 for broadcast } } // Convert binary back to dotted-decimal var networkAddress = ''; var broadcastAddress = ''; for (var j = 0; j < 4; j++) { networkAddress += parseInt(networkBinary.substring(j * 8, j * 8 + 8), 2) + (j < 3 ? '.' : ''); broadcastAddress += parseInt(broadcastBinary.substring(j * 8, j * 8 + 8), 2) + (j < 3 ? '.' : ''); } // Calculate usable IPs var totalAddresses = Math.pow(2, (32 – cidrMask)); var usableIPs = totalAddresses – 2; if (usableIPs < 0) usableIPs = 0; // Handle /31 and /32 cases // Generate Subnet Mask (dotted-decimal) var subnetMask = ''; for (var k = 0; k < 4; k++) { subnetMask += parseInt(maskBinary.substring(k * 8, k * 8 + 8), 2) + (k < 3 ? '.' : ''); } // Display Result resultDiv.innerHTML = 'CIDR Calculation Results:' + '
' + 'Network Address: ' + networkAddress + " + 'Broadcast Address: ' + broadcastAddress + " + 'Subnet Mask: ' + subnetMask + " + 'Usable Host IPs: ' + usableIPs + ' (' + totalAddresses + ' Total)' + '
'; // Optional: Success styling for result resultDiv.style.backgroundColor = 'var(–success-green)'; }

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