This is an original planning guide. It is not a vendor manual, an official standard, or legal advice. Example addresses use documentation ranges. Review every command before you paste it.
What this hour is for
Overlap means two prefixes that both claim the same address. The checker compares ranges you paste. It does not scan a network.
The usual causes are a summary that swallowed a more specific, a VPN that joined two sites both using 10.0.0.0/8, and a copied template. Write which prefix stays and which one moves.
Fixing the spreadsheet is this hour. Renumbering a live VLAN is a later change with its own rollback. Do not advertise either prefix until the overlap is gone.
You only paste prefixes you are allowed to hold.
The change you can finish
Apply nothing during the writing of the note. When the lab window opens, it is one change on the device named in the ticket. Read the draft, then the device, then decide.
Rollback is the file or the revision you saved first. You do not invent a rollback after the only copy is gone.
- Paste the prefixes from your own plan.
- Read the pair the checker names.
- Decide which block moves.
- Update the plan.
- Do not renumber a live VLAN in this hour.
- Stop after one pair.
Mistakes that outlive the command
How to Find and Fix Overlapping Subnets goes wrong in a small set of ways. The list is here so the ticket can forbid them before the window.
- Renumbering production to test the checker.
- Ignoring a VPN overlap because both sides are private.
- Advertising the overlapping prefix.
- Pasting someone else's plan.
- Changing two sites in one window.
Decisions to write down
One decision for How to Find and Fix Overlapping Subnets: The prefixes are yours. One pair is named. One side will move. The live change waits. And nothing was advertised.
Checks before you call it done
Confirm this before you call the change done: One overlap is written. The surviving prefix is named. The moving prefix is named. No device was renumbered. And a second person agrees. If one of those fails, stop.
Before you leave the device
How to Find and Fix Overlapping Subnets ends when the hostname on the device matches the hostname in the ticket. If it does not, you are on the wrong box. Stop. A command that succeeded on the wrong box is not progress.
The draft stays a draft until a person has read the names in it. A generator does not log into the device and does not know which interface is the uplink. You delete a line that names a device, a zone, or a prefix you did not write down.
0.0.0.0/0 is not kept because a sample included it. A source you can say is the source you keep. 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32 are documentation. They belong in the note. They are not prefixes you type onto a production device because they appeared in a draft.
prompt-for-this is a reminder that a secret is missing. It is not a password, a key, a community string, or a pre-shared key you save in the ticket or in git. The real secret stays in the store.
One change is the window. A VLAN and a routing protocol and a VPN do not share it. If two things move and the result is wrong, you will not know which one did it. The next feature waits for its own page.
If the same step fails twice, stop for the day. The failure is the result. Adding a forum command beside a half-applied config produces a device nobody can rebuild. Restore the copy you took first.
A second person reads the device name and the change aloud. If they cannot find them in the ticket, the note is not done. Silence is not agreement. Then leave the device alone.
This page is a planning guide. It is not a vendor manual, not an official standard, and not a permission to touch a network you do not administer. The pillar guide owns the wider design. This page owns one lab proof.
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