VLSM Subnet Planner
Plan variable-length subnets across several different host-count requirements from one base network.
Variable Length Subnet Masking is what you use when your networks are different sizes — which they always are. Rather than carving a block into equal pieces and wasting most of them, you allocate each subnet only as much space as it needs, largest first.
How to use it
- Enter the base network in CIDR notation.
- List the required host counts, one per line, largest first.
- Press Plan subnets and read the allocation.
What to take away
Largest first is not a style preference
Allocating big blocks first keeps the remaining space contiguous. Start small and you fragment the range so badly that a later large subnet will not fit at all.
Add two to every host count
The network and broadcast addresses are not usable. A subnet for 30 hosts needs 32 addresses, which is a /27.
Leave room to grow
Sizing exactly to today's count guarantees a renumbering project later. Round up a step where the address space allows.
Frequently asked questions
What does VLSM mean in practice?
Using different prefix lengths within one address block, so a 500-host site and a 2-host point-to-point link do not both consume a /24.
How do I pick the prefix for a given host count?
Find the smallest power of two that is at least hosts plus two. 30 hosts needs 32 addresses, so /27; 100 hosts needs 128, so /25.
Why do point-to-point links use /30 or /31?
A /30 gives exactly two usable addresses, which is all a link between two routers needs. A /31 gives two with no broadcast address at all, which modern equipment supports and which halves the waste.
Where this fits in the course
This security tool pairs with Path 02 — Networking — see all 26 learning paths.
It runs entirely in your browser: nothing is uploaded, no account is needed, and it is free.
Related
- Subnet Calculator — IPv4 CIDR → network, broadcast, host range, mask, and host count.
- CIDR Overlap Checker — Check whether two CIDR ranges overlap — catches conflicting firewall/routing rules before they cause an outage.
- Raw HTTP Request Builder — Build a raw HTTP request from method, path, headers, and body — see exactly what goes over the wire.
- Security Header Analyzer — Paste HTTP response headers and get a graded report on the security headers present and missing.