# A VLSM allocation, worked end to end

> A full variable-length subnet allocation for a realistic network: sizing each segment, sorting largest-first, assigning the actual addresses, and accounting for the space left over.

Source: https://ronutz.com/en/learn/vlsm-worked-example  
Updated: 2026-06-27  
Related tools: https://ronutz.com/en/tools/cidr

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## The scenario

You are given `172.16.0.0/22` and have to fit four segments inside it:

- **Sales**: 500 hosts
- **Engineering**: 200 hosts
- **Link A**: a point-to-point router link, 2 hosts
- **Link B**: a second point-to-point link, 2 hosts

A `/22` holds 1024 addresses (`172.16.0.0` to `172.16.3.255`). The conceptual rules are covered in [VLSM](https://ronutz.com/en/learn/vlsm); here we run them all the way through.

## Step 1: size each segment

Find the smallest block that holds each host count, remembering the two reserved addresses (network and broadcast) in every ordinary block:

- 500 hosts needs 9 host bits (2⁹ − 2 = 510 usable), a `/23`.
- 200 hosts needs 8 host bits (254 usable), a `/24`.
- 2 hosts needs 2 host bits (2 usable), a `/30`.
- 2 hosts needs another `/30`.

Round up to the next power of two: 500 does not fit a `/24` (254 usable), so it takes a `/23`.

## Step 2: sort largest first

Order the segments by block size, biggest first, so each lands on a clean boundary:

1. Sales, `/23` (512 addresses)
2. Engineering, `/24` (256)
3. Link A, `/30` (4)
4. Link B, `/30` (4)

## Step 3: assign the addresses

Allocate in that order, each block starting where the previous one ended:

```
/23  172.16.0.0  – .1.255   (510 usable)  ← Sales
/24  172.16.2.0  – .2.255   (254 usable)  ← Engineering
/30  172.16.3.0  – .3.3     (2 usable)    ← Link A
/30  172.16.3.4  – .3.7     (2 usable)    ← Link B
```

Sales fills the first half of the block. Engineering takes the next `/24`. The two router links take four addresses each at the start of the final `/24`.

## Step 4: account for what is left

The four segments use 512 + 256 + 4 + 4 = 776 addresses, leaving 248 free, from `172.16.3.8` to `172.16.3.255`. That is 75.8% utilization, with a contiguous tail of free space for the next link or a small subnet, still aligned for future summarization.

## Checking your work

This is exactly the allocation the [CIDR calculator](https://ronutz.com/en/#cidr) produces in VLSM mode: enter `172.16.0.0/22` as the parent and the four host counts, and it returns the same networks, masks, ranges, and the 75.8% utilization figure, computed entirely in your browser. If a segment does not fit, it is listed separately rather than silently dropped.