VLSM Calculator — Variable Length Subnet Masking

Last reviewed: May 2026

Variable Length Subnet Masking (VLSM) lets you assign different prefix lengths to different subnets within the same network. Instead of wasting addresses by giving every segment a /24, VLSM sizes each subnet to exactly what it needs — conserving IP space and simplifying routing tables.

What Problem Does VLSM Solve?

Before VLSM, classful networking forced every subnet to have the same mask. If you had three departments needing 50, 25, and 5 hosts respectively, you'd assign three /26 blocks — wasting 127 addresses in the smallest department. VLSM allocates a /26 for 50 hosts, a /27 for 25 hosts, and a /29 for 5 hosts.

Worked Example: 192.168.10.0/24

Requirements: Engineering (50 hosts), Finance (25 hosts), IoT (5 hosts), Point-to-point link (2 hosts).

Step 1 — Sort by host count, largest first:

1. Engineering: 50 hosts
2. Finance: 25 hosts
3. IoT: 5 hosts
4. P2P link: 2 hosts

Step 2 — Find the smallest prefix that fits each requirement:

Step 3 — Assign sequential address blocks:

Total addresses used: 108 out of 256. A fixed /26 approach would have consumed 256 addresses with more waste.

VLSM Rules to Follow

• Always allocate largest subnets first to avoid fragmentation
• Each subnet's starting address must be on a boundary divisible by its block size (e.g., /26 starts on multiples of 64)
• Subnets must not overlap — use a conflict checker to verify before deploying
• Leave gaps for future growth when possible

VLSM Host Count Formula

To find the right prefix for n hosts: find the smallest k where 2^k − 2 ≥ n, then the prefix is /( 32 − k ).