IP addressing
Calculated locallyVLSM Planner
Divide one parent network into right-sized, non-overlapping subnets and see exactly how each allocation was calculated.
Learn VLSM planning step by step
Follow largest-first allocation, overlap prevention, and infrastructure reservation guidance in the complete article.
Build your address plan
Enter the parent network and usable hosts needed by each subnet.
Subnet requirements
0 networks
| Name | Required hosts | Purpose |
|---|
Generated plan
Click a network to copy it or open it in the IPv4 calculator.
| # | Name | Requested | With growth | Network | Mask | Usable range | Broadcast | Capacity | Unused |
|---|
Interactive address-space map
Blocks are proportional to the addresses consumed inside the parent network.
Engineer Insights
Validation, utilization, growth, and design observations.
Step through the allocation
Watch the planner assign networks from largest to smallest.
Step 1
Learn mode
How VLSM is calculated
The planner follows the same sequence an engineer uses by hand.
Sort largest first
Large requirements are allocated before small ones to reduce fragmentation.
Add growth reserve
The chosen percentage is added to each requested host count and rounded up.
Find the smallest fit
The planner chooses the smallest power-of-two block with enough usable addresses.
Use valid boundaries
Each block begins at the next address aligned to its subnet size.
Traditional IPv4 sizing
usable hosts = 2^(32 − prefix) − 2Example: 50 hosts
/27 = 30 usable → too small
/26 = 62 usable → selectedWhy largest-first allocation?
Several small subnets can leave gaps that are too small or incorrectly aligned for a later large subnet. Largest-first allocation keeps the plan compact and predictable.
How does growth reserve work?
A 20% reserve changes a 100-host requirement to 120 before sizing. A /25 supports 126 usable addresses, so /25 is selected.
What is special about /31?
On an RFC 3021 routed point-to-point link, both addresses can be used and there is no traditional network or broadcast address.