VLSM calculator

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Subnets sized from host counts, no overlaps.

Parent block

IPv4

Sized for

120 of 256 allocated · 46.9%

salesengineeringadminserversfree

CIDR	Usable range	Usable	Spare
192.168.1.0/26	192.168.1.1 – 192.168.1.62	62	19%
192.168.1.64/27	192.168.1.65 – 192.168.1.94	30	0%
192.168.1.96/28	192.168.1.97 – 192.168.1.110	14	29%
192.168.1.112/29	192.168.1.113 – 192.168.1.118	6	17%

Export

CSV

Terraform export is for AWS: choose AWS under "Sized for".

name,cidr,first_usable,last_usable,usable,hosts_needed,sized_for
sales,192.168.1.0/26,192.168.1.1,192.168.1.62,62,50,Standard
engineering,192.168.1.64/27,192.168.1.65,192.168.1.94,30,30,Standard
admin,192.168.1.96/28,192.168.1.97,192.168.1.110,14,10,Standard
servers,192.168.1.112/29,192.168.1.113,192.168.1.118,6,5,Standard

Standard: 2 addresses reserved per subnet.

Free blocks

136 addresses

192.168.1.120/298
192.168.1.128/25128

About this tool

Runs in: your browser
Your input sent to our servers: Nothing
Kept on this device: nothing

How to use it

01Enter the parent block, such as 192.168.1.0/24.
02Edit the rows, or add one per subnet, with the hosts each needs.
03Choose the platform under Sized for and the order the plan follows.
04Export the plan as CSV, or as Terraform when it is sized for AWS.

Questions

What is VLSM?

Variable-length subnet masking: each subnet gets the smallest prefix that fits its host count, instead of one size for all.

Why do cloud subnets need more room?

AWS reserves 5 addresses in every subnet, Azure 5 and Google Cloud 4, against 2 on a standard network. Pick your platform under "Sized for" and the plan accounts for it.

Why does "Largest first" pack better?

Placing the biggest subnets first keeps every subnet on its natural boundary with no gaps between them. "Keep my order" follows your row order and can leave gaps, which show up under free blocks.