VLSM Subnet Planner
Split a network into subnets sized by how many hosts each one needs — the most efficient CIDR for each, with no overlaps
Uses 148 of 256 addresses (58%).
Subnets and the hosts each needs
Plan
| Name | Hosts | Subnet | Mask | Usable range | Broadcast | Usable | Spare |
|---|---|---|---|---|---|---|---|
| Engineering | 60 | 192.168.10.0/26 | 255.255.255.192 | 192.168.10.1 – 192.168.10.62 | 192.168.10.63 | 62 | 2 |
| Sales | 28 | 192.168.10.64/27 | 255.255.255.224 | 192.168.10.65 – 192.168.10.94 | 192.168.10.95 | 30 | 2 |
| Guest Wi-Fi | 25 | 192.168.10.96/27 | 255.255.255.224 | 192.168.10.97 – 192.168.10.126 | 192.168.10.127 | 30 | 5 |
| Servers | 12 | 192.168.10.128/28 | 255.255.255.240 | 192.168.10.129 – 192.168.10.142 | 192.168.10.143 | 14 | 2 |
| Router link | 2 | 192.168.10.144/30 | 255.255.255.252 | 192.168.10.145 – 192.168.10.146 | 192.168.10.147 | 2 | 0 |
Still free for later: 192.168.10.148/30, 192.168.10.152/29, 192.168.10.160/27, 192.168.10.192/26
Each subnet gets the smallest block whose usable hosts cover what you asked for: block size minus the network and broadcast addresses (or minus 5 with AWS / Azure rules). Subnets are placed largest first, so every block starts on its own boundary and nothing overlaps. Need to combine or clean up a list of ranges instead? Try CIDR Merge.
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About this tool
The VLSM Subnet Planner splits a network into subnets of different sizes based on how many hosts each needs. Enter the address space (for example 192.168.10.0/24) and a list like Engineering 60, Sales 28, Servers 12, Router link 2, and it gives every subnet the smallest block that fits, with its CIDR, subnet mask, usable host range and broadcast address — without overlaps and with the remaining free blocks listed for later. VLSM (variable-length subnet masking) avoids wasting addresses on a one-size-fits-all mask. Subnets are placed largest first, so every block starts on its own boundary. Switch on AWS/Azure rules to account for the 5 addresses those clouds reserve in every subnet and their /16–/28 size limits.
When to use it
- →Planning subnets for an office, campus or lab network
- →Laying out VPC subnets in AWS or Azure with reserved addresses counted
- →Solving subnetting homework and certification exercises (CCNA, Network+)
- →Checking whether a set of subnets fits in an address block
Tips
- ◆Each subnet needs room for its network and broadcast address: 30 hosts fit a /27, 31 hosts need a /26.
- ◆Leave growth room — ask for a little more than today's host count.
- ◆Point-to-point router links only need 2 hosts, which fits a /30.
- ◆Copy the plan as a table and paste it into a spreadsheet or your docs.
Frequently asked questions
What is VLSM?
Variable-length subnet masking: splitting a network into subnets of different sizes instead of equal ones, so each subnet gets just the addresses it needs.
How many hosts fit in each subnet size?
A block of 2^n addresses has 2^n − 2 usable hosts (network and broadcast are reserved): /26 = 62, /27 = 30, /28 = 14, /29 = 6, /30 = 2. In AWS and Azure subnets, 5 addresses are reserved instead.
Why are the biggest subnets placed first?
Placing blocks from largest to smallest keeps each one aligned to its own size, so they pack together without gaps or overlaps.
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