function calculateSubnet() {
var ipAddressInput = document.getElementById("ipAddress").value;
var cidrPrefixInput = document.getElementById("cidrPrefix").value;
var errorMessageDiv = document.getElementById("errorMessage");
var subnetResultsDiv = document.getElementById("subnetResults");
errorMessageDiv.style.display = "none";
subnetResultsDiv.style.display = "none";
// Validate IP Address
var ipParts = ipAddressInput.split('.');
if (ipParts.length !== 4) {
errorMessageDiv.textContent = "Invalid IP Address format. Please use dotted decimal (e.g., 192.168.1.0).";
errorMessageDiv.style.display = "block";
return;
}
var ipInts = [];
for (var i = 0; i < ipParts.length; i++) {
var part = parseInt(ipParts[i], 10);
if (isNaN(part) || part 255) {
errorMessageDiv.textContent = "Invalid IP Address octet. Each part must be between 0 and 255.";
errorMessageDiv.style.display = "block";
return;
}
ipInts.push(part);
}
// Validate CIDR Prefix
var cidr = parseInt(cidrPrefixInput, 10);
if (isNaN(cidr) || cidr 32) {
errorMessageDiv.textContent = "Invalid CIDR Prefix. Must be a number between 0 and 32.";
errorMessageDiv.style.display = "block";
return;
}
// Convert IP to 32-bit integer
var ipAddressInt = (ipInts[0] << 24) | (ipInts[1] << 16) | (ipInts[2] << 8) | ipInts[3];
// Calculate Subnet Mask (32-bit integer)
var subnetMaskInt = (0xFFFFFFFF <>> 24) & 0xFF) + '.' +
((intVal >>> 16) & 0xFF) + '.' +
((intVal >>> 8) & 0xFF) + '.' +
(intVal & 0xFF);
}
// Convert 32-bit integer to binary string
function intToBinary(intVal) {
var binary = (intVal >>> 0).toString(2); // Convert to unsigned 32-bit binary
while (binary.length 2) { // Normal subnet, subtract network and broadcast
usableHosts -= 2;
} else { // For /31 and /32, totalHosts is 2 or 1, so usableHosts is 0 or 1 respectively
usableHosts = 0; // Should not happen with the above conditions, but as a fallback
}
// Display Results
document.getElementById("outputSubnetMaskDecimal").textContent = intToIp(subnetMaskInt);
document.getElementById("outputSubnetMaskBinary").textContent = intToBinary(subnetMaskInt);
document.getElementById("outputNetworkAddress").textContent = intToIp(networkAddressInt) + " /" + cidr;
document.getElementById("outputBroadcastAddress").textContent = intToIp(broadcastAddressInt);
if (cidr === 31 || cidr === 32) {
document.getElementById("outputFirstUsableIP").textContent = "N/A (No usable range)";
document.getElementById("outputLastUsableIP").textContent = "N/A (No usable range)";
} else {
document.getElementById("outputFirstUsableIP").textContent = intToIp(firstUsableIPInt);
document.getElementById("outputLastUsableIP").textContent = intToIp(lastUsableIPInt);
}
document.getElementById("outputTotalHosts").textContent = totalHosts;
document.getElementById("outputUsableHosts").textContent = usableHosts;
subnetResultsDiv.style.display = "block";
}
Understanding the Subnet Mask and IP Subnetting
In the world of computer networking, an IP address identifies a unique device on a network. However, for devices to communicate efficiently and for networks to be managed effectively, IP addresses are divided into two main parts: the network portion and the host portion. This division is determined by something called a Subnet Mask.
What is a Subnet Mask?
A subnet mask is a 32-bit number that separates the IP address into the network address and the host address. It works by performing a bitwise AND operation with an IP address. In simple terms, where the subnet mask has '1's, the corresponding bits in the IP address represent the network. Where it has '0's, the corresponding bits represent the host.
Subnet masks are typically represented in dotted-decimal notation, just like IP addresses. Common examples include:
255.255.255.0 (for a /24 network)
255.255.0.0 (for a /16 network)
255.0.0.0 (for a /8 network)
In binary, 255.255.255.0 looks like 11111111.11111111.11111111.00000000. The first 24 bits are '1's, indicating the network portion, and the last 8 bits are '0's, indicating the host portion.
The Role of CIDR (Classless Inter-Domain Routing)
Before CIDR, IP addresses were categorized into classes (A, B, C, D, E) with fixed subnet masks. This led to inefficient use of IP addresses. CIDR, introduced in 1993, revolutionized IP addressing by allowing for more flexible network sizing.
Instead of fixed classes, CIDR uses a CIDR prefix (also known as a prefix length or subnet prefix) to denote the number of bits in the IP address that represent the network portion. This is written as a slash followed by a number, like /24, /16, or /8.
An IP address 192.168.1.0/24 means that the first 24 bits (192.168.1) are the network address, and the last 8 bits (.0) are for host addresses.
This /24 prefix directly corresponds to the subnet mask 255.255.255.0.
The CIDR prefix makes it much easier to understand the size of a network and its associated subnet mask.
Key Subnetting Concepts Explained
When you perform subnetting, several important addresses and ranges are derived:
Network Address: This is the first address in a given subnet. It's used to identify the network itself and cannot be assigned to a host. All host bits are set to '0'.
Example: For 192.168.1.100/24, the Network Address is 192.168.1.0.
Broadcast Address: This is the last address in a given subnet. Packets sent to this address are delivered to all devices within that specific subnet. It cannot be assigned to a host. All host bits are set to '1'.
Example: For 192.168.1.100/24, the Broadcast Address is 192.168.1.255.
First Usable IP: This is the first IP address that can be assigned to a host device within the subnet. It is always one greater than the Network Address.
Example: For 192.168.1.0/24, the First Usable IP is 192.168.1.1.
Last Usable IP: This is the last IP address that can be assigned to a host device within the subnet. It is always one less than the Broadcast Address.
Example: For 192.168.1.0/24, the Last Usable IP is 192.168.1.254.
Total Hosts: This is the total number of IP addresses available within a subnet, calculated as 2^(32 - CIDR prefix).
Example: For a /24 network, 2^(32-24) = 2^8 = 256 total hosts.
Usable Hosts: This is the number of IP addresses that can actually be assigned to devices. It's typically Total Hosts - 2 (subtracting the Network Address and Broadcast Address).
Example: For a /24 network, 256 - 2 = 254 usable hosts.
Note: For /31 networks (point-to-point links), there are 2 total hosts and 2 usable hosts. For /32 networks (single host), there is 1 total host and 1 usable host.
Why is Subnetting Important?
Network Segmentation: Subnetting allows a large network to be divided into smaller, more manageable segments. This improves security by isolating traffic and limits the impact of network issues to smaller areas.
Improved Performance: By reducing the size of broadcast domains, subnetting can decrease network congestion and improve overall performance.
Efficient IP Address Utilization: Especially with CIDR, subnetting helps in allocating IP addresses more efficiently, preventing wastage of valuable IP address space.
Organizational Structure: It helps in organizing networks logically, making it easier for administrators to manage and troubleshoot.
Using the Subnet Mask Calculator
Our Subnet Mask Calculator simplifies the process of understanding subnetting. Simply enter an IP address and its corresponding CIDR prefix, and the calculator will instantly provide you with all the essential details: the subnet mask in both decimal and binary, the network address, broadcast address, the range of usable IP addresses, and the total and usable host counts. This tool is invaluable for network administrators, students, and anyone looking to deepen their understanding of IP networking.