Calculate Subnet

Subnet Calculator

Enter an IP address and CIDR prefix to see subnet details.

function calculateSubnet() { var ipAddressStr = document.getElementById("ipAddressInput").value; var cidrStr = document.getElementById("cidrInput").value; var resultDiv = document.getElementById("result"); resultDiv.innerHTML = ""; // Clear previous results // 1. Validate IP Address Input var ipParts = ipAddressStr.split('.'); if (ipParts.length !== 4) { resultDiv.innerHTML = "Invalid IP Address format. Please use A.B.C.D."; return; } var ipOctets = []; for (var i = 0; i < 4; i++) { var octet = parseInt(ipParts[i], 10); if (isNaN(octet) || octet 255) { resultDiv.innerHTML = "Invalid IP Address octet: '" + ipParts[i] + "'. Must be between 0 and 255."; return; } ipOctets.push(octet); } // 2. Validate CIDR Prefix Input var cidr = parseInt(cidrStr, 10); if (isNaN(cidr) || cidr 32) { resultDiv.innerHTML = "Invalid CIDR Prefix. Must be an integer between 0 and 32."; return; } // Helper function to convert a 32-bit integer to dotted decimal IP format function intToIp(intVal) { return ((intVal >>> 24) & 0xFF) + '.' + ((intVal >>> 16) & 0xFF) + '.' + ((intVal >>> 8) & 0xFF) + '.' + (intVal & 0xFF); } // 3. Convert IP Address to a 32-bit integer var ipInt = (ipOctets[0] << 24) | (ipOctets[1] << 16) | (ipOctets[2] << 8) | ipOctets[3]; // 4. Calculate Subnet Mask (as a 32-bit integer) // -1 in 32-bit binary is all ones (111…111) // Shifting left by (32 – cidr) bits creates the mask // e.g., for /24, 32-24=8. -1 << 8 results in 111…11100000000 (255.255.255.0) var subnetMaskInt = -1 << (32 – cidr); // 5. Calculate Network Address (bitwise AND of IP and Subnet Mask) var networkAddressInt = ipInt & subnetMaskInt; // 6. Calculate Wildcard Mask (bitwise NOT of Subnet Mask) // This flips all bits of the subnet mask. // e.g., for 255.255.255.0, it becomes 0.0.0.255 var wildcardMaskInt = ~subnetMaskInt; // 7. Calculate Broadcast Address (bitwise OR of Network Address and Wildcard Mask) var broadcastAddressInt = networkAddressInt | wildcardMaskInt; // 8. Convert all 32-bit integers back to dotted decimal format var networkAddress = intToIp(networkAddressInt); var broadcastAddress = intToIp(broadcastAddressInt); var subnetMaskDotted = intToIp(subnetMaskInt); var wildcardMaskDotted = intToIp(wildcardMaskInt); // 9. Calculate Number of Hosts and Usable Hosts var totalHosts = Math.pow(2, (32 – cidr)); var usableHostsCount = 0; var firstUsableHost = "N/A"; var lastUsableHost = "N/A"; if (cidr < 31) { // For /31 and /32, there are no "usable" hosts in the traditional sense (excluding network/broadcast) usableHostsCount = totalHosts – 2; firstUsableHost = intToIp(networkAddressInt + 1); lastUsableHost = intToIp(broadcastAddressInt – 1); } else if (cidr === 31) { // /31 has 2 addresses, often used for point-to-point links. Both are assignable. // For this calculator, we'll stick to the "network/broadcast excluded" definition for usable hosts. usableHostsCount = 0; firstUsableHost = "N/A (Point-to-point)"; lastUsableHost = "N/A (Point-to-point)"; } else if (cidr === 32) { // /32 is a single host address, no network/broadcast distinction usableHostsCount = 0; firstUsableHost = "N/A (Single host)"; lastUsableHost = "N/A (Single host)"; } // 10. Display Results var output = "

Subnet Details:

"; output += "IP Address: " + ipAddressStr + ""; output += "CIDR Prefix: /" + cidr + ""; output += "Subnet Mask: " + subnetMaskDotted + ""; output += "Wildcard Mask: " + wildcardMaskDotted + ""; output += "Network Address: " + networkAddress + ""; output += "Broadcast Address: " + broadcastAddress + ""; output += "First Usable Host: " + firstUsableHost + ""; output += "Last Usable Host: " + lastUsableHost + ""; output += "Total Addresses: " + totalHosts + ""; output += "Usable Hosts: " + usableHostsCount + ""; resultDiv.innerHTML = output; }

Understanding Subnetting: A Comprehensive Guide

Subnetting is a fundamental concept in computer networking that involves dividing a larger network into smaller, more manageable subnetworks (subnets). This process is crucial for efficient IP address management, improved network performance, and enhanced security.

What is Subnetting and Why is it Important?

At its core, subnetting takes a single IP network and breaks it down into multiple smaller networks. Each of these smaller networks is called a subnet. The primary reasons for subnetting include:

  • Efficient IP Address Utilization: Prevents wasting large blocks of IP addresses when only a few are needed for a specific segment.
  • Reduced Network Congestion: By creating smaller broadcast domains, broadcast traffic is confined to its respective subnet, reducing overall network congestion.
  • Improved Security: Subnets can be isolated from each other, allowing administrators to apply different security policies and restrict access between segments.
  • Simplified Network Management: Organizing devices into logical groups makes troubleshooting and administration easier.
  • Enhanced Performance: Smaller networks mean less traffic for routers to process, leading to faster data transmission.

How Subnetting Works: The Basics

Every device on a network needs an IP address, which consists of two parts: the network portion and the host portion. The subnet mask is used to distinguish these two parts. In subnetting, you "borrow" bits from the host portion of an IP address to create more network bits, thereby creating more subnets.

Key Components:

  • IP Address: A unique numerical label assigned to each device connected to a computer network. It's typically represented in dotted-decimal notation (e.g., 192.168.1.100).
  • Subnet Mask: A 32-bit number that separates the network address from the host address within an IP address. It's also represented in dotted-decimal (e.g., 255.255.255.0). Where there's a '1' in the subnet mask, it's part of the network address; where there's a '0', it's part of the host address.
  • CIDR Prefix (Classless Inter-Domain Routing): A more concise way to represent the subnet mask. It's denoted by a slash followed by the number of network bits (e.g., /24 means the first 24 bits are for the network).
  • Network Address: The first address in a subnet, where all host bits are zero. It identifies the subnet itself and cannot be assigned to a host.
  • Broadcast Address: The last address in a subnet, where all host bits are one. Packets sent to this address are delivered to all devices within that subnet. It cannot be assigned to a host.
  • Usable Host Addresses: The IP addresses between the network address and the broadcast address that can be assigned to devices on the network.
  • Wildcard Mask: The inverse of the subnet mask. It's used in access control lists (ACLs) to specify a range of IP addresses.

Manual Subnet Calculation Example

Let's take an example: You have an IP address 192.168.1.100 with a CIDR prefix of /26.

  1. Convert IP Address to Binary:
    • 192 = 11000000
    • 168 = 10101000
    • 1 = 00000001
    • 100 = 01100100
    So, 11000000.10101000.00000001.01100100
  2. Determine Subnet Mask from CIDR /26:

    A /26 prefix means the first 26 bits are network bits (1s) and the remaining 6 bits (32-26) are host bits (0s).

    Binary: 11111111.11111111.11111111.11000000

    Dotted Decimal: 255.255.255.192

  3. Calculate Network Address (IP AND Subnet Mask): 
    IP:    11000000.10101000.00000001.01100100
Mask:  11111111.11111111.11111111.11000000
-----------------------------------------
Net:   11000000.10101000.00000001.01000000

    Dotted Decimal Network Address: 192.168.1.64

  4. Calculate Broadcast Address:

    Take the Network Address and set all host bits (the last 6 bits in this case) to 1.

    Net:   11000000.10101000.00000001.01000000
Host bits to 1:                      111111
-----------------------------------------
Bcast: 11000000.10101000.00000001.01111111

    Dotted Decimal Broadcast Address: 192.168.1.127

  5. Calculate First and Last Usable Host:
    • First Usable Host: Network Address + 1 = 192.168.1.65
    • Last Usable Host: Broadcast Address – 1 = 192.168.1.126
  6. Calculate Total and Usable Hosts:
    • Total Addresses: 2^(32 - CIDR) = 2^(32 - 26) = 2^6 = 64
    • Usable Hosts: Total Addresses – 2 (for network and broadcast) = 64 - 2 = 62
  7. Wildcard Mask:

    Inverse of the subnet mask.

    Subnet Mask: 255.255.255.192

    Wildcard Mask: 0.0.0.63 (255.255.255.255 - 255.255.255.192)

Using the Subnet Calculator

Our Subnet Calculator simplifies this complex process. Simply enter:

  1. IP Address: The IP address of any device within the network you want to analyze (e.g., 192.168.1.100).
  2. CIDR Prefix: The number of network bits (e.g., 24 for a 255.255.255.0 mask).

Click "Calculate Subnet," and the tool will instantly provide the Network Address, Broadcast Address, Subnet Mask, Wildcard Mask, and the range of usable host IP addresses, along with the total number of addresses and usable hosts.

Common CIDR Prefixes and Subnet Masks

CIDR Prefix Subnet Mask Total Addresses Usable Hosts
/8255.0.0.016,777,21616,777,214
/16255.255.0.065,53665,534
/24255.255.255.0256254
/25255.255.255.128128126
/26255.255.255.1926462
/27255.255.255.2243230
/28255.255.255.2401614
/29255.255.255.24886
/30255.255.255.25242
/31255.255.255.25420 (Point-to-point)
/32255.255.255.25510 (Single host)

Mastering subnetting is essential for anyone working with computer networks, from IT professionals to network administrators. This calculator serves as a valuable tool to quickly and accurately perform subnet calculations, aiding in network design, troubleshooting, and optimization.

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