VLSM Calculator

What is VLSM?

Variable Length Subnet Masking (VLSM) divides one IPv4 network into multiple subnets of different sizes, each sized to its own host requirement, instead of splitting a network into equal-sized pieces. It makes efficient use of a limited address block by giving a 100-host segment a large subnet and a 10-host segment a small one, rather than forcing every subnet to be the same size.

Why use VLSM?

Fixed-size subnetting (FLSM) wastes addresses whenever requirements differ in size: if the largest network needs a /25, splitting the whole block into equal /25s forces every other, much smaller network to consume a /25 too. VLSM instead sizes each subnet to what it actually needs, leaving more address space free for future growth — visible directly in the Free Address Space section above.

How this calculator works

Enter a base network and a name and required host count for each subnet you need. The calculator sorts requirements from largest to smallest, then allocates each one the smallest standard IPv4 subnet that provides enough usable host addresses, placing each block immediately after the previous one on a valid CIDR boundary. All of the underlying IPv4 arithmetic (prefixes, masks, network and broadcast addresses, usable ranges) is the same calculation engine used by the Subnet Calculator.

Why largest networks are allocated first

Every valid IPv4 subnet size is a power of two, so a larger block's address count is always an exact multiple of any smaller block's. Placing the largest requirement first, immediately followed by the next largest, guarantees every later block starts already aligned to its own CIDR boundary, with zero wasted padding between subnets — the Allocation order shown in the Address Plan table above reflects this. Allocating smallest-first does not have this guarantee and can leave unusable alignment gaps.

How to choose a prefix for a host requirement

Find the smallest prefix whose usable address count (total addresses minus 2, for the network and broadcast addresses) is at least the number of hosts needed. 50 hosts needs a /26 (62 usable), since a /27 only provides 30. 100 hosts needs a /25 (126 usable), since a /26 only provides 62. The Why this prefix? section above walks through this exact contrast for every requirement in your plan.

Planning for growth

A growth reserve inflates each LAN requirement before sizing its subnet, so a 100-host requirement with 20% reserve plans for 120 hosts, not 100. The Growth Headroom section above shows, for every allocated network, how many additional planned hosts still fit before a larger prefix would be needed — the networks with the least headroom are the first candidates for a future resize.

VLSM vs. FLSM

FLSM (Fixed Length Subnet Masking) splits a network into equal-sized subnets, which is simple but wastes addresses when requirements vary. VLSM allows every subnet to have its own prefix length, matched to its own requirement, at the cost of needing a more careful allocation process, which is exactly what this calculator automates.

VLSM vs. CIDR

CIDR is the notation and addressing model (network/prefix, such as /24) that makes variable-length subnets possible in the first place. VLSM is the planning technique of applying CIDR recursively within one network to create differently sized subnets. Every VLSM allocation is expressed in CIDR notation, but not every use of CIDR involves VLSM planning.

Common VLSM mistakes

• Sizing a subnet from total addresses instead of usable addresses (forgetting the network and broadcast address are reserved).
• Allocating smallest-first, which can leave unusable alignment gaps between subnets.
• Not reserving any growth headroom, forcing a disruptive re-address later.
• Assuming every subnet must start with the same host as its gateway; a suggested gateway is a convention, not a requirement.

/31 point-to-point note

A /31 remains a valid network prefix, for example 10.0.0.0/31. Under RFC 3021 point-to-point semantics, neither of its two addresses is reserved as the traditional network or broadcast address, so both can be used as link endpoints — the reason point-to-point links (such as router-to-router WAN links) can use a /31 instead of the larger /30 a normal subnet would need. This calculator only allocates /31 when a requirement is explicitly marked point-to-point in the advanced row options; ordinary LAN requirements are never unexpectedly given a /31.

IPv4 only, by design

This VLSM planner is IPv4-first: it plans variable-length subnets from host requirements, a model built around the usable-hosts-per-subnet arithmetic that's central to IPv4 addressing. It does not currently plan IPv6 prefixes.

Worked example

Click Load example above to see this exact configuration calculated instantly. Base network 192.168.10.0/24 with four requirements:

Users needs 100 hosts, so the calculator selects /25 (126 usable), the smallest block that fits, since /26 only provides 62. The remaining three requirements are placed the same way, each immediately after the previous block. After all four subnets, 192.168.10.240/28 (16 addresses) remains free inside the /24.

Related tools

Need equal-size subnets instead of variable-length ones? Use the Subnet Splitter. Need details about one allocation? Use the Subnet Calculator. Checking whether two networks conflict? Use the Subnet Overlap Checker. Converting a range or aggregating CIDRs? Use the CIDR & IP Range Converter. Reducing a routing table to the fewest prefixes? Use the Route Summarization Calculator.