🗺️ VLAN Subnet Mapper – Plan and Document Your Network
The VLAN Subnet Mapper is a comprehensive network planning tool that helps engineers and administrators assign IP subnets to VLANs, detect overlapping allocations, validate gateway addresses, and export structured documentation — all in the browser without installing any software.
What Is a VLAN-to-Subnet Mapping?
In a modern enterprise or data-center network, each VLAN (Virtual LAN) typically corresponds to a unique IP subnet. For example, VLAN 10 might represent the HR department and use the subnet 192.168.10.0/24, while VLAN 20 hosts servers on 10.0.20.0/24. Maintaining a clear, accurate mapping between VLAN IDs and their subnets prevents routing confusion, firewall misconfigurations, and IP address conflicts.
Key Features
🔢 Live Subnet Calculation
Each row instantly computes the network address, broadcast, first and last usable host, subnet mask, wildcard mask, and usable host count as you type.
⚡ Auto-Assign Mode
Provide a parent block (e.g., 172.16.0.0/20) and a prefix length (e.g., /24) to instantly slice the block into equal subnets and populate all VLAN rows.
🚨 Overlap Detection
The tool checks every pair of subnets and flags conflicting rows with a warning badge so you catch planning mistakes before deployment.
✅ Gateway Validation
Optionally enter a default gateway per VLAN. The tool verifies the gateway falls within the subnet's usable host range and flags it if not.
📊 Address Space Analysis
Specify a parent address block to see what percentage has been allocated, how many addresses remain, and where the unallocated gaps are.
📥 CSV Export
Download the full VLAN-to-subnet mapping as a CSV file for spreadsheets, or copy a plain-text table for wikis and documentation.
Subnet Calculation Formulas
All calculations use standard 32-bit IPv4 bitwise arithmetic. Given a CIDR notation like 192.168.10.0/24:
- Subnet mask —
/24→ the top 24 bits are 1, lower 8 bits are 0 →255.255.255.0 - Network address — IP AND subnet mask →
192.168.10.0 - Broadcast — network address OR (NOT subnet mask) →
192.168.10.255 - First usable host — network address + 1 →
192.168.10.1 - Last usable host — broadcast − 1 →
192.168.10.254 - Usable hosts — 2^(32 − prefix) − 2 →
254 - Wildcard mask — NOT subnet mask →
0.0.0.255(used in Cisco ACLs)
VLAN ID Rules (IEEE 802.1Q)
| VLAN Range | Type | Notes |
|---|---|---|
0 | Reserved | IEEE 802.1Q priority tagged frames |
1 | Default VLAN | Native/default on most switches |
2–1001 | Normal Range | Standard user VLANs, supported by all 802.1Q switches |
1002–1005 | Cisco Reserved | Legacy FDDI and Token Ring (Cisco-specific) |
1006–4094 | Extended Range | Requires VTP Transparent mode or VTP v3 |
4095 | Reserved | Reserved by IEEE 802.1Q, not assignable |
Auto-Assign Mode: How It Works
When auto-assign is enabled, the tool divides a parent address block into equal subnets sequentially:
Parent: 10.0.0.0/20, Prefix: /24, VLANs: 4
→ VLAN 1: 10.0.0.0/24 (hosts: 10.0.0.1 – 10.0.0.254)
→ VLAN 2: 10.0.1.0/24 (hosts: 10.0.1.1 – 10.0.1.254)
→ VLAN 3: 10.0.2.0/24 (hosts: 10.0.2.1 – 10.0.2.254)
→ VLAN 4: 10.0.3.0/24 (hosts: 10.0.3.1 – 10.0.3.254)If the parent block is exhausted before all rows are filled, the tool stops assigning and displays a warning on the remaining rows.
Overlap Detection Algorithm
Two subnets A and B overlap if:
A.networkAddress < B.broadcastAddress
AND B.networkAddress < A.broadcastAddressThe tool checks every pair of entered subnets and annotates conflicting rows with an error badge. This prevents situations where two VLANs inadvertently share address space, which would cause routing loops or duplicate-IP problems.
Common Use Cases
- Enterprise LAN design — map department VLANs (HR, Finance, IT, Guest) to separate subnets for access control and traffic segmentation
- Data center planning — assign subnets to server, storage, and management VLANs with clear documentation
- Campus networks — plan multi-floor VLAN allocations that span several switches and need consistent Layer 3 addressing
- Auditing existing plans — import a current VLAN list and validate for overlaps, missing gateways, or address space waste
- Documentation generation — export a complete VLAN-to-subnet table as CSV for inclusion in network diagrams or runbooks