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VLSM Subnet Planner

Enter a base network and your VLAN host requirements. The planner assigns the most efficient subnet to each VLAN using Variable Length Subnet Masking and outputs ready-to-paste Cisco IOS configuration.

Base Network
Base Network Address
CIDR Prefix
Enter a valid IPv4 network address (e.g. 10.0.0.0)
VLANs / Subnets Required
Total hosts required exceed the available addresses in the base network. Increase the base network size or reduce VLAN requirements.
VLSM Plan — Optimally Assigned Subnets
Base Network
Total VLANs
Addresses Used
Addresses Free
Address Space Utilisation
0%
VLAN NameHosts Req.CIDRNetworkSubnet MaskFirst HostLast HostBroadcastUsable
Cisco IOS Interface Config
How to Plan a VLSM Network

Start with a base network large enough to hold all required VLANs or subnets. Add each VLAN name and the number of hosts required. The planner sorts larger requirements first and assigns the smallest suitable subnet to each group, reducing wasted address space.

VLSM is useful when different departments or device groups need different subnet sizes. A server VLAN may need 20 addresses, a camera VLAN may need 80 addresses and a point-to-point link may need only two. Fixed-size subnetting would waste addresses; VLSM allocates a more suitable block for each need.

The generated Cisco IOS interface configuration is a starting point. Review interface names, VLAN IDs, gateway addresses and site standards before applying any configuration to production equipment.

VLSM FAQ

Why are larger VLANs allocated first? Allocating the largest subnets first helps avoid fragmentation and makes it more likely that all requirements will fit inside the base network.

Should I leave spare addresses? Yes. Real networks grow. Leave capacity for new devices, printers, wireless access points, cameras, temporary devices and future VLANs.

Can this replace network design review? No. It helps with address planning, but routing, DHCP, DNS, firewall policies, redundancy and documentation still need proper review.

About VLSM Subnet Planning

Variable Length Subnet Masking (VLSM) is the technique of dividing an IP address space into subnets of different sizes based on the actual number of hosts each network segment needs. Without VLSM, you would assign the same size subnet to every VLAN regardless of how many hosts it contains, wasting large blocks of address space on small segments. VLSM solves this by allowing each subnet to be sized independently.

The correct VLSM allocation order is to assign subnets from largest to smallest host requirement. This prevents fragmentation of the address space and ensures the most efficient packing of subnets within the base network. This planner handles that ordering automatically. You enter your VLANs in any order and the tool sorts them correctly before assigning subnets.

The Cisco IOS Interface Config output generates the ip address and no shutdown commands for each VLAN interface, using the first usable host address as the gateway IP. This is ready to paste into a Layer 3 switch or router configuration without any manual calculation.

Worked Example

Base network: 10.0.0.0/24
VLAN: Sales 50 hosts → /26 (62 usable) Network: 10.0.0.0/26 Gateway: 10.0.0.1 Broadcast: 10.0.0.63 VLAN: IT 20 hosts → /27 (30 usable) Network: 10.0.0.64/27 Gateway: 10.0.0.65 Broadcast: 10.0.0.95 VLAN: Mgmt 10 hosts → /28 (14 usable) Network: 10.0.0.96/28 Gateway: 10.0.0.97
Why does VLSM assign subnets largest-first?
Large subnets must start on address boundaries that are multiples of their block size. Assigning them first ensures they fit cleanly at the start of the address space. If you assigned a small subnet first, the next large subnet might need to skip addresses to align, wasting space.
What CIDR prefix does the tool assign for a given host count?
It selects the smallest subnet that accommodates the required number of hosts. For 50 hosts it assigns /26 (62 usable). For 25 hosts it assigns /27 (30 usable). For 10 hosts it assigns /28 (14 usable). This minimises wasted address space.

Common Use Cases

  • 🏢
    New office network design. Plan subnets for multiple departments (Sales, IT, Servers, Management, VoIP, Guest) within a single allocated block, then paste the Cisco config directly into your switch.
  • 🎓
    CCNA exam preparation. VLSM design questions are a core part of the CCNA exam. Use this tool to verify your manual calculations and understand the correct assignment order.
  • 📋
    Network documentation. Generate a complete IP address plan with all subnet details in a structured table for network documentation and IPAM records.
  • 🔧
    Cisco switch configuration. The Cisco IOS output gives you the exact interface vlan, ip address and no shutdown commands to paste into a Layer 3 switch during a network build.