VLSM (Variable Length Subnet Masking) Calculator

ANA›Life Services Authority›National Calculator Authority›VLSM (Variable Length Subnet Masking) Calculator

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VLSM (Variable Length Subnet Masking) Calculator

Efficiently allocate IP address space by dividing a network into subnets of varying sizes based on your host requirements. Enter a base network and the number of hosts needed for each subnet.

Base Network Address (e.g. 192.168.1.0/24)

Subnet Host Requirements (one per line, e.g. "Sales 50" or just "50") Sales 50 HR 25 IT 10 Management 5

Calculate Subnets

...

function vlsCalc() { const resultDiv = document.getElementById('vls-result');

// --- Parse base network --- const networkInput = document.getElementById('vls-network').value.trim(); const cidrMatch = networkInput.match(/^(\d{1,3}.\d{1,3}.\d{1,3}.\d{1,3})\/(\d{1,2})$/); if (!cidrMatch) { resultDiv.innerHTML = '❌ Invalid network format. Use CIDR notation, e.g. 192.168.1.0/24'; return; }

const baseIP = cidrMatch[1]; const basePrefixLen = parseInt(cidrMatch[2]);

if (basePrefixLen 30) { resultDiv.innerHTML = '❌ Prefix length must be between 0 and 30.'; return; }

// Validate base IP octets const octets = baseIP.split('.').map(Number); if (octets.some(o => o 255)) { resultDiv.innerHTML = '❌ Invalid IP address octets.'; return; }

// Convert IP to 32-bit integer function ipToInt(ip) { const parts = ip.split('.').map(Number); return ((parts[0] >> 0; }

function intToIp(n) { n = n >>> 0; return [(n >>> 24) & 0xFF, (n >>> 16) & 0xFF, (n >>> 8) & 0xFF, n & 0xFF].join('.'); }

function prefixToMask(prefix) { if (prefix === 0) return 0; return (0xFFFFFFFF >> 0; }

function maskToWildcard(mask) { return (~mask) >>> 0; }

// --- Parse subnet requirements --- const subnetLines = document.getElementById('vls-subnets').value.trim().split('\n').filter(l => l.trim() !== ''); if (subnetLines.length === 0) { resultDiv.innerHTML = '❌ Please enter at least one subnet requirement.'; return; }

const subnets = []; for (let i = 0; i ❌ Invalid entry on line ' + (i+1) + ': "' + line + '". Use format "Name Hosts" or just "Hosts".'; return; } const name = match[1].trim() || ('Subnet ' + (i+1)); const hosts = parseInt(match[2]); if (hosts 16777214) { resultDiv.innerHTML = '❌ Host count on line ' + (i+1) + ' must be between 1 and 16,777,214.'; return; } subnets.push({ name, hosts }); }

// --- VLSM Algorithm --- // Sort subnets by host count descending (largest first) subnets.sort((a, b) => b.hosts - a.hosts);

// Calculate required prefix for each subnet // Hosts needed = 2^(32-prefix) - 2 >= hosts required // So 32-prefix >= ceil(log2(hosts+2)) // prefix >> 0;

if (actualBaseNet !== baseNetInt) { resultDiv.innerHTML = '❌ The IP address is not a valid network address for /' + basePrefixLen + '. Did you mean ' + intToIp(actualBaseNet) + '/' + basePrefixLen + '?'; return; }

// Total available addresses in base network const totalAvailable = Math.pow(2, 32 - basePrefixLen);

// Check if all subnets fit let totalRequired = 0; const subnetPrefixes = []; for (const s of subnets) { const prefix = requiredPrefix(s.hosts); if (prefix ❌ Subnet "' + s.name + '" requires /' + prefix + ' which is larger than the base network /' + basePrefixLen + '.'; return; } if (prefix > 30) { resultDiv.innerHTML = '❌ Subnet "' + s.name + '" requires too many hosts for a valid subnet (max 2 hosts per /30).'; return; } const size = Math.pow(2, 32 - prefix); totalRequired += size; subnetPrefixes.push(prefix); }

if (totalRequired > totalAvailable) { resultDiv.innerHTML = '❌ Insufficient address space. Required: ' + totalRequired + ' addresses, Available: ' + totalAvailable + ' addresses in /' + basePrefixLen + '.'; return; }

// Allocate subnets let currentIP = actualBaseNet; const results = [];

for (let i = 0; i >> 0; currentIP = aligned;

const networkAddr = currentIP; const broadcastAddr = (currentIP + size - 1) >>> 0; const firstHost = (networkAddr + 1) >>> 0; const lastHost = (broadcastAddr - 1) >>> 0; const usableHosts = size - 2;

results.push({ name: s.name, hostsRequired: s.hosts, prefix, size, networkAddr: intToIp(networkAddr), broadcastAddr: intToIp(broadcastAddr), firstHost: intToIp(firstHost), lastHost: intToIp(lastHost), subnetMask: intToIp(mask), wildcardMask: intToIp(wildcard), usableHosts, cidr: intToIp(networkAddr) + '/' + prefix });

currentIP = (broadcastAddr + 1) >>> 0; }

// Calculate address utilization const usedAddresses = results.reduce((sum, r) => sum + r.size, 0); const utilization = ((usedAddresses / totalAvailable) * 100).toFixed(2); const wastedAddresses = usedAddresses - subnets.reduce((sum, s) => sum + s.hosts, 0);

// Build result table let html = '### VLSM Allocation Results '; html += 'Base Network: ' + intToIp(actualBaseNet) + '/' + basePrefixLen; html += '  |  Total Addresses: ' + totalAvailable.toLocaleString(); html += '  |  Used: ' + usedAddresses.toLocaleString(); html += '  |  Utilization: ' + utilization + '%

';

html += ''; html += ''; html += 'Subnet Name'; html += 'Hosts Req.'; html += 'Prefix'; html += 'Network Address'; html += 'Subnet Mask'; html += 'Wildcard Mask'; html += 'First Host'; html += 'Last Host'; html += 'Broadcast'; html += 'Usable Hosts'; html += 'Total Addr.'; html += '';

results.forEach((r, idx) => { const bg = idx % 2 === 0 ? '#f9f9f9' : '#fff'; html += ''; html += '' + r.name + ''; html += '' + r.hostsRequired.toLocaleString() + ''; html += '/' + r.prefix + ''; html += '' + r.cidr + ''; html += '' + r.subnetMask + ''; html += '' + r.wildcardMask + ''; html += '' + r.firstHost + ''; html += '' + r.lastHost + ''; html += '' + r.broadcastAddr + ''; html += '' + r.usableHosts.toLocaleString() + ''; html += '' + r.size.toLocaleString() + ''; html += ''; });

html += '';

// Summary stats html += ''; html += 'Summary: '; html += results.length + ' subnets allocated  |  '; html += 'Total hosts required: ' + subnets.reduce((s,r) => s + r.hosts, 0).toLocaleString() + '  |  '; html += 'Total usable hosts allocated: ' + results.reduce((s,r) => s + r.usableHosts, 0).toLocaleString() + '  |  '; html += 'Address waste (overhead): ' + wastedAddresses.toLocaleString() + ' addresses  |  '; const remaining = totalAvailable - usedAddresses; html += 'Remaining unallocated: ' + remaining.toLocaleString() + ' addresses'; html += '';

resultDiv.innerHTML = html; }

#### Formula

Required Prefix Length:

prefix = 32 - ⌈log₂(hosts_required + 2)⌉ subnet_size = 2^(32 - prefix) usable_hosts = subnet_size - 2 network_address = base_ip & subnet_mask broadcast_address = network_address + subnet_size - 1 first_host = network_address + 1 last_host = broadcast_address - 1 subnet_mask = 0xFFFFFFFF << (32 - prefix) wildcard_mask = ~subnet_mask

The +2 accounts for the network address and broadcast address which cannot be assigned to hosts. Subnets are sorted largest-to-smallest and allocated sequentially to minimize wasted address space.

#### Assumptions & References

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References

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