This worked scenario uses documentation-style private addresses. It is a planning example, not a claim about your network. Check your existing IP address management records before choosing a parent range.
1. Count everything that needs an address
Suppose the approved parent allocation is 192.168.10.0/24. You need space for 50 staff devices, 20 voice devices and 10 management devices. These counts already include gateways, static reservations and the growth allowance chosen for this example. If your counts exclude those items, add them before sizing.
2. Choose the smallest conventional LAN subnet that fits
A conventional IPv4 LAN reserves its network and broadcast addresses. A /26 has 64 total addresses and 62 host addresses; a /27 has 32 and 30; a /28 has 16 and 14. Allocate the largest requirement first so small blocks do not fragment the parent space.
| VLAN purpose | Required hosts | Subnet | Conventional host range | Broadcast |
|---|---|---|---|---|
| Staff | 50 | 192.168.10.0/26 | 192.168.10.1–192.168.10.62 | 192.168.10.63 |
| Voice | 20 | 192.168.10.64/27 | 192.168.10.65–192.168.10.94 | 192.168.10.95 |
| Management | 10 | 192.168.10.96/28 | 192.168.10.97–192.168.10.110 | 192.168.10.111 |
The allocations consume 112 of the parent block’s 256 addresses. The remaining address space is .112 through .255. It is not one aligned CIDR block: exact coverage is 192.168.10.112/28 plus 192.168.10.128/25.
3. Reproduce the plan with the calculators
- Open the VLSM calculator. Enter parent 192.168.10.0/24 and host requirements 50, 20 and 10, one per line.
- Use the IPv4 subnet calculator to check each block’s host range and broadcast address.
- In IP Range to CIDR, enter 192.168.10.112 through 192.168.10.255 to describe the exact remaining allocation.
4. Check the operational details before applying it
- Confirm that the parent range does not overlap VPN clients, other sites, cloud networks or existing routed networks.
- Reserve gateway and infrastructure addresses explicitly; the host range is not automatically the DHCP pool.
- A VLAN separates broadcast domains, but access policy requires appropriate routing and firewall controls. Subnet arithmetic alone does not establish isolation.
- Check device and provider reservations. Some platforms reserve more addresses than the conventional network/broadcast pair.
- Record VLAN IDs, gateways, DHCP scope exclusions, DNS options, routing and rollback steps in your change plan.
Continue learning
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