IPv4 subnet calculator
Calculate the network, broadcast, usable address range and host count from an IPv4 address with a CIDR prefix or subnet mask.
VLSM subnet planner
Plan IPv4 subnets by host count, inspect usable ranges and free blocks, and export the allocations as CSV.
One subnet per line: name = hosts, or just a host count. Up to 100 subnets.
Unallocated CIDR blocks
Download allocation as CSV
About this tool
Divide a parent network among departments or device groups with different capacity requirements.
How to use
Enter an IPv4 parent network with a CIDR prefix, then add up to 100 lines of name=hosts, such as LAN=100. A host count alone also works. Subnets are allocated from largest to smallest; review usable host ranges, spare capacity and remaining CIDR blocks, then copy or download the CSV.
Ordinary subnets reserve a network and broadcast address by default. Enable the /31 and /32 option to use those prefixes for two-host and single-host requests. Insufficient space produces an error rather than a partial plan. No network configuration is changed.
Common questions
Are network and broadcast addresses reserved?
Ordinary subnets reserve both. Enable /31 and /32 support for two-host and single-host requests. Allocation runs largest first and stops if the parent network lacks space.
Why is a /24 full when my requested hosts total less than 254?
Each subnet is rounded up to a power-of-two block after reserving its network and broadcast addresses. Two requests for 100 hosts each use two /25 blocks and consume the entire /24. Use a larger parent network or reduce the requests; unused host slots inside a subnet are not unallocated blocks.
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