VLSM subnet planner

What VLSM means

Variable length subnet masking means not every subnet in a network has to be the same size. A server network of 200 hosts and a point-to-point link of 2 hosts do not need the same block, and giving both a /24 wastes most of the address space. A VLSM plan gives each one the smallest block that holds it.

This planner takes a network, such as 10.20.0.0/22, and a list of needs, such as Servers: 200 or Guests: /26. It sorts them, gives each the right size, places them without gaps, and shows what is left over as free blocks you can use later.

How the sizes and the order are chosen

A subnet needs room for its hosts plus two addresses that cannot be given to a host, the network address and the broadcast address. So 200 hosts need 202 addresses, and the next power of two is 256, which is a /24. Fifty hosts need 52, which rounds up to 64, a /26.

The planner places the biggest subnets first. A block must start on an address that is a multiple of its own size, so putting small blocks first can leave a hole that no larger block can use. Largest first keeps every block aligned and the free space in as few pieces as possible.

When the plan does not fit

A plan that is too big is not an error. The subnets that fit are placed, the ones that did not are listed with how much room they needed and how much was left in the largest free block, and the page says how large a network would hold everything. Plan the same needs in a /24 instead of a /22 and you see at once which teams do not fit.

Cloud networks and IPv6

Cloud providers keep addresses in every subnet for themselves. Amazon VPC and Azure keep five, Google Cloud keeps four. Choose the provider under More options and the planner adds those addresses to each need, so a subnet for 60 hosts becomes a /25 in AWS, not a /26. Providers also set the smallest subnet they allow, and the planner raises smaller needs to that size and tells you.

IPv6 is different. Subnets are not sized by host count, because a /64 already holds more addresses than any link will use, and features such as SLAAC need a /64. Every IPv6 subnet here gets a /64 and the plan counts subnets. The equal split can also line each subnet up on a hex digit boundary, so the addresses read cleanly.

Export and privacy

The plan can be copied as CSV, as JSON, or as Terraform aws_subnet or azurerm_subnet blocks. With JavaScript on, the plan is worked out in your browser and nothing you type is sent to us. Without JavaScript the form sends your network and needs to our server in the page address. The subnet calculator shows the details of any one block from the plan.