VLSM Subnet Planner & Calculator

List the subnets you need and how many hosts each has. Get an aligned plan with free space left over.

IPv4 & IPv6 · your network layout never leaves the browser

Address spaceReserved per subnet

AWS reserves the first 4 and the last address; subnets /16–/28.

Subnets you need (5)

Plan

8,832 of 65,536 addresses allocated13.47%

Name	Need	Range	Usable
public-a	250	10.0.32.1 – 10.0.32.254	251
public-b	250	10.0.33.1 – 10.0.33.254	251
private-a	4,000	10.0.0.1 – 10.0.15.254	4,091
private-b	4,000	10.0.16.1 – 10.0.31.254	4,091
database	60	10.0.34.1 – 10.0.34.126	123

Free space left

10.0.34.128/2510.0.35.0/2410.0.36.0/2210.0.40.0/2110.0.48.0/2010.0.64.0/1810.0.128.0/17

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Frequently Asked Questions

Variable Length Subnet Masking gives each subnet a size that fits its host count, instead of equal-sized subnets, so less address space is wasted.

Placing large blocks first keeps every subnet aligned to its boundary and avoids fragmentation.

Yes. Choose AWS, Azure or GCP and it adds their reserved addresses to each subnet's size.

Yes — copy it as a Terraform locals map of subnet names to CIDRs.