Why does it warn when the DHCP pool includes the gateway?

Because the scope will eventually lease the router's own address to a workstation, and the subnet loses its default route. It is the commonest way a working network is broken by a configuration change, and the symptom — 'the internet is down for some people' — rarely points at the cause.

Can I use the same subnet on two VLANs?

Not within one routing table. Two broadcast domains sharing a subnet gives hosts addresses that look local and are not reachable. The exception is different VRFs, where overlapping space is the point, and the validator allows that.

Why does it warn about VLAN 1?

It is the default on most switches and usually the untagged native VLAN on trunks. Carrying user traffic there is a standing finding in security reviews, and moving off it costs little.

What are VLANs 1002 to 1005?

Reserved by convention for legacy Token Ring and FDDI defaults. Many switches still refuse to create them, so a plan that uses them will fail at configuration time rather than at design time.

No. The allocation, the validation and the exports all run in your browser. Nothing about your network leaves the machine.

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VLAN + Subnet Designer

Name your VLANs and say how many devices each holds. It allocates the subnets, proposes gateways and DHCP ranges, then checks the whole plan for the mistakes that take a network down.

Local toolYour network plan stays in your browser. Nothing you enter is uploaded, logged, or sent to any server.

Parent network

Growth allowance

Gateway position

DHCP pool

VLANs

What it checks, and why each one matters

Allocation is the easy half. These are the faults the validator looks for, and what each one actually does to a live network.

Duplicate VLAN ID. Within a switching domain a VLAN ID identifies exactly one broadcast domain. Two definitions means two designs fighting over one.
Overlapping subnets. Hosts get addresses that look local and are not reachable. The symptom is intermittent and extremely hard to trace.
DHCP range covering the gateway. The scope eventually leases the router's own address to a workstation and the subnet loses its default route. This is the single most common way a working network is broken by a configuration change.
DHCP range outside the subnet, or including the network or broadcast address. The scope hands out addresses that cannot work.
Gateway outside its subnet, or set to the network or broadcast address.
Insufficient capacity, and a warning when a subnet is above 85% full, because there is no room for a visitor's laptop or a replacement printer.
VLAN 1 carrying user traffic, and VLAN IDs in the 1002–1005 range that many switches reserve.

Gateways default to the first usable address. Last usable is offered because plenty of sites use that convention instead; what matters is consistency, not which one.

Related tools

VLSM Designer does the addressing without the VLAN layer. Subnet Calculator inspects any one prefix. VLAN Planner documents VLANs you already have.

VLAN + Subnet Designer

VLANs

Validation

Address space

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