VLSM Planner
Enter a base network and a list of subnets with the hosts each one needs. I compute the optimal VLSM allocation (aligned blocks, largest first) with mask, range, broadcast and utilization. All in your browser.
Base network utilization
VLSM allocation
| Subnet | CIDR | Mask | Network | First host | Last host | Broadcast | Usable | Req. |
|---|
VLSM gives each subnet the smallest block that fits it and places them
largest first from the base network, so they stay
aligned with no overlap. No subnet with hosts uses
/31 or /32 (it always keeps a network and broadcast).
Runs locally in your browser · no sign-up · nothing leaves your browser
How it works
The planner takes a base network in CIDR format (for example 192.168.1.0/24) and a list of subnets with the hosts each one needs, and builds the full VLSM assignment: CIDR, mask, network address, first and last host, broadcast and usable hosts for every block, plus the total utilization of the base network and the addresses left free.
The algorithm is classic VLSM (variable-length subnet masking): it sorts the subnets from largest to smallest, finds for each one the longest prefix that satisfies 2^(32−prefix) − 2 ≥ hosts and assigns the blocks sequentially from the start of the base network. Going largest-first keeps every block naturally aligned to its size, with no overlap and no alignment gaps. No subnet with hosts ever gets a /31 or /32: the network and broadcast addresses are always reserved.
Example: splitting 192.168.1.0/24 across 4 subnets
- Requirements: Sales 50 hosts, Support 20, IT 10, Link 2. For 50 hosts the smallest block that works is a /26 (62 usable); for 20, a /27 (30); for 10, a /28 (14); for 2, a /30 (2).
- They are assigned largest-first from the base: Sales
192.168.1.0/26, Support192.168.1.64/27, IT192.168.1.96/28and Link192.168.1.112/30. - Total assigned: 64 + 32 + 16 + 4 = 116 of 256 addresses (45%); 140 remain free from 192.168.1.116 for growth.