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// CISCO & INFRA · BROWSER

VLAN Planning Helper

Check a whole VLAN plan: every subnet and gateway, duplicate IDs and overlapping subnets, then print the IOS configuration for all of the VLANs.

CISCO · IN YOUR BROWSER
One VLAN per line: id, name, subnet (CIDR), gateway, optional description. Blank lines and lines starting with # are ignored.

Runs in your browser; nothing you type leaves this page. Too long to share as a link.

HISTORY

Your last 5 runs appear here for 7 days, on this device only.

RESULT

Fill in the form and run the tool; the result appears here.

VLAN Planning Helper checks a whole VLAN plan, one VLAN per line as ID, name, subnet, gateway and an optional description. It flags duplicate IDs, overlapping subnets and gateways outside the subnet, and generates the matching vlan and SVI (switch virtual interface) configuration lines.

How to read the result

The table shows VLAN ID, name, subnet and gateway per entry, with any duplicate ID, overlapping subnet or out-of-range gateway noted against the entries involved, so you catch it before it reaches a device rather than after. Subnets are IPv4 in CIDR form; a line that cannot be read is skipped and listed under the table.

Defender tips

  • Segment by trust level, not just by floor or department: guest, IoT, servers and management each belong in their own VLAN with explicit rules between them, not a flat network with VLANs used only for broadcast domain size.
  • Keep management VLANs off any path routed from user-facing networks; a management interface reachable from a general VLAN removes most of the value segmentation was meant to provide.
  • Reserve and document VLAN ID ranges by purpose (a block for guest, a block for IoT, and so on) before you start assigning them ad hoc; retrofitting a numbering scheme onto a live network is far more disruptive than planning one first.

FAQ

What is the maximum number of VLANs?

4094 usable IDs under the 802.1Q standard, with a small number reserved at each end of the range by convention (0 and 4095 are reserved outright, and 1 and 1002 to 1005 carry special meaning on Cisco gear).

Does it check for overlap with subnets I have already entered?

Yes. Each run compares every entry with every other one in the plan, so both duplicate VLAN IDs and overlapping subnets are flagged wherever they appear, not only between adjacent lines.

Does it configure inter-VLAN routing for me?

It generates the VLAN and SVI configuration lines; whether and how you route between VLANs (an ACL on the SVI, a firewall between zones) is a design decision you still make and apply yourself.

// RELATED FINDING Missing network segmentation: impact, fix and references in the Findings Explorer