VLSM Subnet Planner
Allocate variable-length subnets in a base network from hosts per subnet: largest first, aligned, with network, mask, host range, broadcast and free space.
A plan table with prefix, mask, first and last host and broadcast for every subnet, the addresses used and free, the leftover space as CIDR blocks, and a copyable text version.
Example: In 192.168.10.0/24, Engineering 100 hosts → /25 (.0–.127), Sales 60 → /26 (.128–.191), Mgmt 10 → /28 (.192–.207), a WAN link → /31 (.208–.209); 46 addresses stay free from .210.
Largest first,
on power-of-two boundaries.
How each subnet is sized and placed, and what the plan does with the leftovers.
Sizing
A subnet for h hosts needs the smallest block whose usable addresses cover it: 2^(32 − p) − 2 ≥ h, since the network and broadcast addresses are not usable. Two hosts get a /31 (RFC 3021 point-to-point, no broadcast) and one host a /32. A growth percentage grows every requirement before sizing; the spare addresses each block still carries are listed as waste.
Placement
Blocks are placed largest first, each starting at the next address that is a multiple of its own size — the classic VLSM order, which leaves no unaligned gaps and never overlaps. If a block would run past the base network the page says which subnet did not fit and how much the larger ones had already taken. The space left is reported as aligned CIDR blocks.
Reading the plan
Network, mask, first and last usable host and broadcast are listed per subnet; a router interface takes one of the usable addresses. A base address that is not on its prefix boundary is normalised to the containing network and noted. IPv4 only; IPv6 is planned by nibble boundaries and does not count hosts. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- prefix for h hosts: smallest 2^(32 − p) − 2 ≥ h (/31 for two, RFC 3021; /32 for one); blocks placed largest first on their own size boundary — the standard VLSM procedure; RFC 950 subnetting
Last reviewed 21 September 2026. How results are checked: How we verify.