VLSM Subnet Planner

Frequently asked

How does VLSM differ from fixed-length subnetting?

Fixed-length subnetting gives every VLAN the same mask, so a 2-host WAN link wastes as much space as a 200-host VLAN. VLSM sizes each segment independently, which is what lets a /22 hold a 200-host, a 100-host and a 2-host segment without wasting a /24 on the link.

Why does a 200-host VLAN get a /24 and a 100-host VLAN a /25?

A /24 has 254 usable addresses and a /23 only 510, so 200 hosts fits a /24 exactly. 100 hosts fits a /25 (126 usable) but not a /26 (62 usable), which is why the planner rounds up to the next power of two.

What is the growth headroom for?

It inflates every demand before sizing, so a 100-host VLAN planned with 30% headroom is treated as 130 hosts and gets a /24 instead of a /25. Planning with headroom costs address space now and avoids a re-addressing project later.

When should point-to-point links use a /31?

RFC 3021 allows both addresses of a /31 to be used on a point-to-point link, so a 2-host WAN link needs only two addresses instead of the four a /30 burns. Enable the /31 toggle if both ends of your links support it, which most modern routers do.

Does the planner use any of my addresses remotely?

No. The whole calculation runs in your browser and the tool makes no network request, so your address plan never leaves the machine.