VLSM Subnet Calculator
Divide a network into variable-size subnets by the number of hosts each needs, allocating the largest first, with ranges, broadcast addresses and space used.
Free · No sign-up · Your inputs stay in your browser
Input
Split a network into subnets by size (VLSM)
Output
5 subnets, 48% of the space used
| Subnet | Needed | CIDR | Usable range | Usable | Broadcast |
|---|---|---|---|---|---|
| app servers | 200 | 10.20.0.0/24 | 10.20.0.1 - 10.20.0.254 | 254 | 10.20.0.255 |
| k8s nodes | 120 | 10.20.1.0/25 | 10.20.1.1 - 10.20.1.126 | 126 | 10.20.1.127 |
| databases | 50 | 10.20.1.128/26 | 10.20.1.129 - 10.20.1.190 | 62 | 10.20.1.191 |
| vpn | 30 | 10.20.1.192/27 | 10.20.1.193 - 10.20.1.222 | 30 | 10.20.1.223 |
| management | 10 | 10.20.1.224/28 | 10.20.1.225 - 10.20.1.238 | 14 | 10.20.1.239 |
Largest subnet first, each aligned to its own size, which is the order that wastes the least space. Two addresses per subnet are subtracted for the network and broadcast addresses; cloud providers reserve more (AWS keeps five per subnet), so leave headroom.
Runs in this browser. Nothing you paste is sent to a server.
About this tool
Method, assumptions, practical next steps, and limits.
Why largest first
Blocks must start on a multiple of their size. Allocating the big subnets first keeps alignment gaps out of the address space; allocating small ones first leaves fragments too small for the big ones. That is the standard VLSM procedure from network engineering courses, and it is also how to lay out a VPC.
Cloud subnets lose more
AWS reserves five addresses in every subnet (network, router, DNS, future use and broadcast), Azure five, and Google Cloud four. Size cloud subnets with that in mind, and leave room to grow: renumbering a subnet later means moving everything in it.