VLSM Subnet Calculator

Allocate variable-size IPv4 subnets from host requirements with aligned CIDR, masks, ranges, and remaining space.

Local browser processing

Addresses and calculation inputs stay in this browser. They are not uploaded, resolved, scanned, or sent to analytics.

Address or network

Host requirements, one per line

Result

Enter values and run the tool to see the local result.

What a VLSM calculator does

VLSM assigns different subnet sizes inside one IPv4 network. This calculator sorts host requirements from largest to smallest, chooses the smallest traditional subnet that fits each request, aligns every block, and reports the network, mask, broadcast, host range, and unused remainder.

How to plan VLSM subnets

Enter the parent IPv4 network as a CIDR such as 10.0.0.0/24.
Add one host requirement per line as a number or as a name followed by a comma and number.
Calculate, review the largest-first allocation, and copy the resulting CIDRs and host ranges.

VLSM allocation assumptions

The calculator reserves traditional network and broadcast addresses and therefore uses at least a /30 for a positive host request. It does not infer gateways, VLANs, DHCP reservations, redundancy, growth margin, or point-to-point /31 policy.

Related network tools

IPv4 Subnet Calculator IP Range to CIDR Converter

VLSM planning questions

Why are VLSM requirements sorted largest first?: Allocating the largest power-of-two blocks first reduces alignment gaps and makes it easier to fit smaller networks afterward.
Does the requested host count include network and broadcast?: No. The requested count represents hosts, and the calculator adds the traditional network and broadcast addresses.
What happens when the networks do not fit?: The calculation stops with an explicit insufficient-space error instead of silently placing a subnet outside the parent network.
Can this plan IPv6 subnets?: No. This page applies traditional IPv4 host-count allocation. Use the IPv6 subnet calculator for IPv6 prefix division.

VLSM planning questions

Why are VLSM requirements sorted largest first?

Allocating the largest power-of-two blocks first reduces alignment gaps and makes it easier to fit smaller networks afterward.

Does the requested host count include network and broadcast?

No. The requested count represents hosts, and the calculator adds the traditional network and broadcast addresses.

What happens when the networks do not fit?

The calculation stops with an explicit insufficient-space error instead of silently placing a subnet outside the parent network.

Can this plan IPv6 subnets?

No. This page applies traditional IPv4 host-count allocation. Use the IPv6 subnet calculator for IPv6 prefix division.