Subnet Calculator

Subnet Calculator

Enter an IP address and its CIDR prefix to calculate network details.

Subnet Details:

IP Address:

CIDR Prefix:

Subnet Mask:

Network Address:

Broadcast Address:

First Usable Host:

Last Usable Host:

Total Hosts:

Usable Hosts:

function calculateSubnet() { var ipAddressInput = document.getElementById("ipAddress").value; var cidrPrefixInput = parseInt(document.getElementById("cidrPrefix").value, 10); // Input Validation for IP Address var ipParts = ipAddressInput.split('.'); if (ipParts.length !== 4) { alert("Invalid IP Address format. Please use dotted decimal notation (e.g., 192.168.1.10)."); return; } for (var i = 0; i < 4; i++) { var octet = parseInt(ipParts[i], 10); if (isNaN(octet) || octet 255) { alert("Invalid IP Address octet: '" + ipParts[i] + "'. Each octet must be between 0 and 255."); return; } ipParts[i] = octet; // Convert to number for calculations } // Input Validation for CIDR Prefix if (isNaN(cidrPrefixInput) || cidrPrefixInput 32) { alert("Invalid CIDR Prefix. Please enter a number between 1 and 32."); return; } var originalCidrPrefix = cidrPrefixInput; // Store original CIDR for display and specific logic // Convert CIDR to Subnet Mask var tempCidrPrefix = originalCidrPrefix; // Use a temporary variable for the loop var subnetMaskOctets = [0, 0, 0, 0]; for (var i = 0; i < 4; i++) { var bits = Math.min(tempCidrPrefix, 8); subnetMaskOctets[i] = (256 – Math.pow(2, 8 – bits)); tempCidrPrefix -= bits; } var subnetMask = subnetMaskOctets.join('.'); // Calculate Network Address var networkAddressOctets = []; for (var i = 0; i < 4; i++) { networkAddressOctets[i] = ipParts[i] & subnetMaskOctets[i]; } var networkAddress = networkAddressOctets.join('.'); // Calculate Broadcast Address var broadcastAddressOctets = []; for (var i = 0; i < 4; i++) { broadcastAddressOctets[i] = networkAddressOctets[i] | (255 – subnetMaskOctets[i]); } var broadcastAddress = broadcastAddressOctets.join('.'); // Calculate First Usable Host var firstUsableHost = "N/A"; if (originalCidrPrefix < 31) { // If /31 or /32, no usable hosts in the traditional sense var firstUsableHostOctets = networkAddressOctets.slice(); // Copy array firstUsableHostOctets[3]++; // Increment last octet firstUsableHost = firstUsableHostOctets.join('.'); } else if (originalCidrPrefix === 32) { // For /32, the IP itself is the only "host" firstUsableHost = ipAddressInput; } // Calculate Last Usable Host var lastUsableHost = "N/A"; if (originalCidrPrefix 2) { usableHosts = totalHosts – 2; } else if (originalCidrPrefix === 32) { // /32 has 1 total, 0 usable usableHosts = 0; } else if (originalCidrPrefix === 31) { // /31 has 2 total, 0 usable usableHosts = 0; } // Display Results document.getElementById("outputIpAddress").innerText = ipAddressInput; document.getElementById("outputCidrPrefix").innerText = "/" + originalCidrPrefix; document.getElementById("outputSubnetMask").innerText = subnetMask; document.getElementById("outputNetworkAddress").innerText = networkAddress; document.getElementById("outputBroadcastAddress").innerText = broadcastAddress; document.getElementById("outputFirstUsableHost").innerText = firstUsableHost; document.getElementById("outputLastUsableHost").innerText = lastUsableHost; document.getElementById("outputTotalHosts").innerText = totalHosts; document.getElementById("outputUsableHosts").innerText = usableHosts; }

Understanding IP Subnetting and How This Calculator Works

IP subnetting is a fundamental concept in computer networking that allows a single large network to be divided into smaller, more manageable subnetworks. This division helps improve network performance, enhance security, and efficiently utilize IP addresses.

What is an IP Address?

An IP (Internet Protocol) address is a unique numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication. IPv4 addresses, which this calculator focuses on, are 32-bit numbers typically represented in dotted-decimal notation (e.g., 192.168.1.10).

What is a Subnet Mask?

A subnet mask is a 32-bit number that distinguishes the network portion of an IP address from the host portion. It works in conjunction with an IP address to determine which part identifies the network and which part identifies the specific device (host) within that network. A '1' in the subnet mask indicates a network bit, and a '0' indicates a host bit.

What is CIDR Prefix?

CIDR (Classless Inter-Domain Routing) prefix notation is a more concise way to represent the subnet mask. Instead of writing out the full dotted-decimal subnet mask, CIDR uses a slash followed by a number (e.g., /24). This number indicates how many bits in the IP address are used for the network portion, starting from the left. For example, a /24 prefix means the first 24 bits are for the network, and the remaining 8 bits are for hosts.

Key Subnetting Terms Explained:

  • Network Address: The first address in a subnet. It identifies the network itself and cannot be assigned to a host. All host bits are zero.
  • Broadcast Address: The last address in a subnet. It is used to send data to all devices within that specific network segment. All host bits are one.
  • First Usable Host: The first IP address in a subnet that can be assigned to a device. It is always one greater than the network address, unless the subnet is too small (/31 or /32).
  • Last Usable Host: The last IP address in a subnet that can be assigned to a device. It is always one less than the broadcast address, unless the subnet is too small (/31 or /32).
  • Total Hosts: The total number of IP addresses available within a given subnet, including the network and broadcast addresses. Calculated as 2^(32 – CIDR prefix).
  • Usable Hosts: The number of IP addresses within a subnet that can actually be assigned to devices. This is typically the total hosts minus two (for the network and broadcast addresses). For /31 and /32 subnets, there are no usable hosts.

How the Subnet Calculator Works:

This calculator takes an IP address and a CIDR prefix as input and performs the necessary bitwise operations to determine all the critical subnetting information:

  1. Input Validation: It first checks if the IP address is in a valid format (four octets, each 0-255) and if the CIDR prefix is a number between 1 and 32.
  2. Subnet Mask Calculation: It converts the CIDR prefix into its equivalent dotted-decimal subnet mask.
  3. Network Address: It performs a bitwise AND operation between the IP address and the subnet mask to find the network address.
  4. Broadcast Address: It performs a bitwise OR operation between the network address and the inverted subnet mask to find the broadcast address.
  5. Host Range: It then derives the first and last usable host addresses by adding one to the network address and subtracting one from the broadcast address, respectively, handling special cases for very small subnets.
  6. Host Count: Finally, it calculates the total number of addresses in the subnet and the number of addresses available for hosts.

Example Usage:

Let's say you have an IP address 192.168.1.10 and a CIDR prefix of /24.

  • IP Address: 192.168.1.10
  • CIDR Prefix: /24
  • Subnet Mask: 255.255.255.0
  • Network Address: 192.168.1.0
  • Broadcast Address: 192.168.1.255
  • First Usable Host: 192.168.1.1
  • Last Usable Host: 192.168.1.254
  • Total Hosts: 256
  • Usable Hosts: 254

This means that for the 192.168.1.0/24 network, devices can be assigned IP addresses from 192.168.1.1 to 192.168.1.254.

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