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
ARP maps an IPv4 address to a MAC on a local link. A missing entry means the host did not answer, or you are not on that link. It is not a directory of the company.
Write the address, the MAC, and the interface. A changing MAC for a stable address is a ticket. A stable MAC you do not recognise on your gateway address is a stop, not a thing you spoof.
This page does not tell you to poison a cache, spoof a gateway, or sweep a subnet. Those are attacks. Your job is to read the table on a device you operate.
IPv6 neighbour discovery is the cousin. Do not mix the tables in one note.
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.
- Name the device whose table you will read.
- Read one address you care about.
- Write the MAC and the interface.
- If the gateway MAC changed unexpectedly, stop and escalate.
- Do not sweep.
- Do not spoof.
Mistakes that outlive the command
ARP Explained goes wrong in a small set of ways. The list is here so the ticket can forbid them before the window.
- A subnet sweep to fill the table.
- A spoofed gateway.
- A MAC change ignored on the gateway.
- IPv6 and ARP mixed as one table.
- A neighbour's link.
Decisions to write down
One decision for ARP Explained: The device is yours. One entry was read. A gateway MAC change is escalated. No sweep. And no spoof.
Checks before you call it done
Confirm this before you call the change done: Address, MAC, and interface are written. The device name matches. No scan ran. No spoof was attempted. And an unexpected gateway MAC stopped the work. If one of those fails, stop.
Read one entry on your device.
Do not sweep or spoof.Before you leave the device
ARP Explained 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.
Tools for this guide
This guide does not name a browser tool. The pillar it belongs to links the calculators for the neighbouring work.
Related pillar guides
More in this cluster
The pillar for this page, and the next published pages filed with it.