How to Calculate VLSM

What VLSM is

VLSM means allocating different prefix lengths inside the same parent network based on actual host needs. Instead of giving every VLAN the same size, you assign each subnet the smallest block that fits.

Why fixed-size subnetting wastes addresses

If you used /25 for every segment in a /24 design request, you would quickly run out of address space for small segments while still wasting many addresses in each larger block.

• Users need 100 hosts, so /25 fits (126 usable).
• Management may need 12 hosts, where /28 fits (14 usable).
• Using /25 for management wastes 112 usable addresses.

VLSM calculation method

1. List all subnet requirements by usable host count.
2. Sort from largest host requirement to smallest.
3. For each requirement, choose the smallest prefix where usable hosts are sufficient.
4. Allocate that prefix on the next valid boundary.
5. Move the cursor to the next available network and repeat.
6. Reserve blocks for infrastructure and growth before finalizing.

Traditional usable hosts = 2^(32 - prefix) - 2
/25 = 126 usable
/26 = 62 usable
/27 = 30 usable
/28 = 14 usable
/30 = 2 usable

Worked example (parent 10.20.0.0/24)

Requirements:

• Users: 100 hosts
• Voice: 50 hosts
• Servers: 25 hosts
• Management: 12 hosts
• Point-to-point: 2 hosts

Step 1: Sort largest to smallest

100, 50, 25, 12, 2.

Step 2: Select prefixes

• 100 hosts -> /25 (126 usable)
• 50 hosts -> /26 (62 usable)
• 25 hosts -> /27 (30 usable)
• 12 hosts -> /28 (14 usable)
• 2 hosts -> /30 (2 usable)

Step 3: Allocate sequentially

1. Users: 10.20.0.0/25 (10.20.0.1-10.20.0.126, broadcast 10.20.0.127)
2. Voice: next block starts at 10.20.0.128 -> 10.20.0.128/26 (10.20.0.129-10.20.0.190, broadcast 10.20.0.191)
3. Servers: next block 10.20.0.192/27 (10.20.0.193-10.20.0.222, broadcast 10.20.0.223)
4. Management: next block 10.20.0.224/28 (10.20.0.225-10.20.0.238, broadcast 10.20.0.239)
5. Point-to-point: next block 10.20.0.240/30 (10.20.0.241-10.20.0.242, broadcast 10.20.0.243)

Next available network after these allocations is 10.20.0.244. Remaining free space can be reserved for future infrastructure.

Allocation summary table

Common VLSM design mistakes

• Allocating small subnets first and fragmenting space.
• Forgetting to align the next network to the selected block size.
• Ignoring growth reserve for user and server segments.
• Allowing accidental overlap between adjacent assignments.
• Mixing up network, usable, and broadcast addresses in documentation.

VLSM versus route summarization

VLSM is an allocation method for designing subnets. Route summarization is a routing advertisement method for collapsing multiple prefixes into fewer route entries.

Use VLSM first to design clean address blocks, then evaluate summaries with the Route Summarization Tool or the article Route Summarization Explained.

VLSM verification checklist

1. Requirements sorted largest to smallest.
2. Each requirement mapped to smallest valid prefix.
3. Each subnet starts on a correct network boundary.
4. No overlap across all assigned blocks.
5. Infrastructure and growth space reserved intentionally.
6. Plan validated in VLSM Planner.

FAQ

Key takeaways