VLSM Subnet Splitter (Variable Length Subnet Masking)
Enter your base network block and comma-separated host requirements. The calculator sorts requirements largest-first to assign non-overlapping subnets cleanly.
Allocated Subnets
| Requested Hosts | Assigned CIDR | Subnet Mask | Usable Range | Broadcast | Capacity |
|---|
How Variable Length Subnet Masking (VLSM) Works
Variable Length Subnet Masking (VLSM) allows network architects to partition a single parent CIDR block (such as 10.0.0.0/22) into multiple child subnets of different sizes, rather than forcing every segment into an identical fixed-length mask. Without VLSM, assigning a full /24 (254 usable IPs) to a two-router WAN link wastes 99% of that address block.
The golden rule of VLSM planning is largest-first allocation. Every valid CIDR block must start on an IP boundary that is an exact multiple of its block size (for example, a /25 block of 128 addresses must start at .0 or .128). By sorting host requirements from largest to smallest before assigning prefixes, each smaller block naturally aligns on valid binary boundaries without leaving unusable gaps or causing overlapping routes.
When sizing cloud subnets in AWS VPC, Azure VNet, or Google Cloud, remember that cloud providers typically reserve 4 to 5 IP addresses per subnet (rather than the traditional 2) for internal DNS, VPC router gateways, and future reservations—so add a small buffer to your host counts when planning cloud infrastructure.