VLSM Subnet Planner

Allocate differently sized IPv4 subnets inside a parent prefix without overlaps or wasted manual arithmetic.

Calculator IP & Subnetting Runs in your browser

Result

Parent
10.20.0.0/20
Subnets
4
Allocated addresses
736
Unallocated addresses
3,360

Largest-first allocation

NameRequired hostsAllocated prefixUsable rangeBroadcast
Users50010.20.0.0/2310.20.0.1 - 10.20.1.25410.20.1.255
Voice12010.20.2.0/2510.20.2.1 - 10.20.2.12610.20.2.127
Servers6010.20.2.128/2610.20.2.129 - 10.20.2.19010.20.2.191
Management2010.20.2.192/2710.20.2.193 - 10.20.2.22210.20.2.223
Allocation list
Users: 10.20.0.0/23
Voice: 10.20.2.0/25
Servers: 10.20.2.128/26
Management: 10.20.2.192/27
VLSM is allocated largest-first to preserve alignment. Reserve growth space deliberately before deploying DHCP scopes or routing policy.

About VLSM Subnet Planner

VLSM divides one IPv4 block into unequal subnets. Enter each segment and its required usable-host count; the planner assigns the smallest conventional subnet that fits and prevents overlap.

Allocation rule

Requests are sorted largest-first. Each allocation includes network and broadcast addresses, is rounded to a power of two and aligned to a valid CIDR boundary.

Operational review

After calculating the plan, compare it with the real VLAN, routing and DHCP design. Record intentional gaps for growth, infrastructure and point-to-point links. A mathematically valid allocation can still be operationally weak when adjacent departments cannot grow, summarisation boundaries are lost, or security zones are mixed. Treat the table as a draft address plan, then assign ownership and document every reserved range before configuration is deployed.

Common use cases

  • Campus VLAN address planning.
  • Preparing an ISP or branch allocation sheet.
  • Checking whether planned networks fit an assigned block.

Edge cases and gotchas

  • Add growth headroom before deployment.
  • /31 point-to-point semantics are intentionally not used for host-count requests.
  • Confirm the final allocation against IPAM, routing summaries, DHCP reservations and security-zone boundaries. Never deploy directly from a calculation without recording ownership, purpose, gateway convention and rollback impact for every assigned prefix.

Frequently asked questions

Why are the requests reordered?
Allocating the largest blocks first preserves alignment and prevents small early allocations from fragmenting the parent network. The displayed name still identifies every request, so the calculated order can be transferred safely into an address-management system after review.
Does the plan reserve future growth automatically?
No. Host counts are treated as current requirements. Add explicit growth subnets or increase requested hosts according to the organisation’s capacity policy before approving the address plan.