Plan summary--
VLSM Subnet Planner
Plan variable-length IPv4 subnets by entering a base network and required host counts. The planner sorts larger subnets first so addresses are assigned more efficiently.
Updated May 2026No signup requiredBuilt for mobile
How to use this calculator
Enter the real measurements or counts you have now, run the calculator, then round the result in the practical direction before buying supplies or making a plan.
Formula or calculation method
Required addresses = hosts + 2 for normal IPv4 subnets
Block size = next power of two that fits required addresses
CIDR prefix = 32 - log2(block size)
Subnets are assigned in order from largest host requirement to smallest.
Practical examples
Example 1
From 192.168.10.0/24, host needs of 100, 50, and 20 can fit as /25, /26, and /27 sized networks.
Example 2
Sorting largest first prevents a large department from being stranded after small subnets consume address space.
Useful assumptions
- This is IPv4 CIDR planning based on RFC 4632-style CIDR concepts.
- For /31 and /32 edge cases, verify how your network equipment treats usable addresses.
- This planner is for addressing design, not firewall or routing policy.
Common mistakes
- Not sorting largest networks first
- Forgetting network and broadcast addresses on normal subnets
- Trying to fit more subnets than the base CIDR can hold
- Using classful assumptions instead of CIDR math
FAQ
VLSM Subnet Planner questions
What does VLSM mean?
VLSM means variable length subnet masking. It lets different subnets use different CIDR sizes.
Why sort largest first?
Large subnets are harder to place later. Assigning them first reduces address fragmentation.
Does this support IPv6?
No. This planner is for IPv4.
Can VLSM replace network design review?
No. Use it for planning, then verify routing, VLANs, DHCP, and firewall rules.
Why can a request fail?
The combined subnet blocks may not fit inside the base network.