VLSM Planner

Allocate variable-length subnets optimally with visual breakdown and CSV export.

Input

Output

Define a starting network, add your subnets and click Plan.

Planning subnets with variable length subnet masking

Variable length subnet masking (VLSM) is the practice of using different prefix lengths within the same address space — assigning a /30 (2 usable hosts) to a point-to-point WAN link while a /22 (1022 usable hosts) serves a large office floor, all carved from the same /16 parent block. Without VLSM, classical subnetting forces every subnet to be the same size, wasting thousands of IP addresses on small segments.

The allocation algorithm sorts requested subnets from largest to smallest, then assigns each one to the next available aligned block. Alignment is critical: a /26 (64-address block) must start on a multiple of 64, otherwise routing entries cannot be aggregated and the block cannot be represented as a single prefix. Getting this wrong by hand is a common source of addressing bugs that only surface weeks later during a route-summarisation project.

Consider a 10.0.0.0/24 block to be split for a small site: the server VLAN needs 50 hosts (/26, 10.0.0.0–10.0.0.63), the management VLAN needs 20 hosts (/27, 10.0.0.64–10.0.0.95), and two WAN uplinks each need just 2 hosts (/30). After allocation, the VLSM calculator shows how much of the /24 is consumed, how many addresses are wasted due to alignment padding, and how much space remains for future growth — all in a colour-coded visual bar.

Overlap detection catches the case where the same address range would be assigned to two different subnets — a mistake that causes silent routing black holes and is nearly impossible to diagnose without examining every entry in the address plan. Once the plan looks correct, export it to CSV for documentation or send it to the Config Generator to produce ready-to-paste Cisco interface and VLAN configurations.

Common use cases

  • Site address planning — allocate a delegated block across VLANs of different sizes before writing any configs.
  • CCNA / CCNP VLSM exercises — verify exam calculations and visualise the allocation tree.
  • ISP prefix delegation — carve customer allocations from a larger aggregate without overlap.
  • Data centre pod design — size server, storage and management subnets independently within a supernet.
  • Migration planning — identify free ranges inside an existing allocation before renumbering a segment.

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