NetKit

VLSM Subnet Planner

Split one block into right-sized VLANs: best-fit allocation for every host count, growth headroom, /31 point-to-point links, plus the leftover free blocks and a utilisation figure.

Input

Input style

The space every segment must fit inside, e.g. 10.20.0.0/22.

SegmentHosts

Sizes every demand with this much headroom.

Which address to propose for the SVI.

Purely a note for your own runbook — the plan does not use it.

Export

5 segment(s) planned

Utilisation

Planned inside 10.20.0.0/22

Base size
1,024

2^10 addresses

Allocated
484

5 block(s)

Utilisation
47.3%

Block space handed out, not host counts

Free
540

4 block(s) left

Failed requests
0

Every segment placed

Gateway convention
First usable

Your note: 10.20.0.1

Allocation

SegmentPrefixUsable rangeGatewayHostsSized forUsableReserved
VLAN10-DATA10.20.0.0/2410.20.0.1 – 10.20.0.25410.20.0.12002002542
VLAN20-VOICE10.20.1.0/2510.20.1.1 – 10.20.1.12610.20.1.11001001262
VLAN30-IOT10.20.1.128/2610.20.1.129 – 10.20.1.19010.20.1.1295050622
VLAN40-MGMT10.20.1.192/2710.20.1.193 – 10.20.1.22210.20.1.1932525302
WAN-CORE-EDGE10.20.1.224/3010.20.1.225 – 10.20.1.22610.20.1.2252222

Free blocks

Remaining space inside the base, as whole CIDR blocks

  • 10.20.1.228/30 · 4 addr
  • 10.20.1.232/29 · 8 addr
  • 10.20.1.240/28 · 16 addr
  • 10.20.2.0/23 · 512 addr

How the plan is chosen

Demands are sorted largest-first and each gets the smallest free block that fits (best fit decreasing), so no segment is starved by a smaller one placed first. 'Sized for' is the requested count after the growth headroom; 'Reserved' counts the network and broadcast addresses a /30-and-longer block gives up — a /31 or /32 gives up none (RFC 3021).

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 page makes no network requests, so your address plan never leaves the machine.

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