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
A /30 has four addresses and two usable hosts in the ordinary habit. A /31 uses both addresses as hosts, which is the point of a link with exactly two routers. The calculator shows the cut. It does not configure a router.
Write which end is which address. A /31 on a LAN of PCs is the wrong tool. A /30 used only because it felt familiar wastes two addresses you may need. Documentation prefixes are the example.
Do not change a live link during the reading. One link. Both ends have to agree before anyone types.
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 two routers.
- Pick /30 or /31 and write why.
- Assign the two addresses.
- Confirm this is not a LAN.
- Do not configure either router.
- Stop.
Mistakes that outlive the command
/30 vs /31 for Point-to-Point Links goes wrong in a small set of ways. The list is here so the ticket can forbid them before the window.
- A /31 on a user VLAN.
- The two ends given different lengths.
- A documentation prefix applied in production.
- Changing the link during the reading.
- A third host squeezed onto the link.
Decisions to write down
One decision for /30 vs /31 for Point-to-Point Links: Both routers are yours. The length matches a two-router link. Both ends use the same length. Nothing was configured. And the LAN stays a LAN.
Checks before you call it done
Confirm this before you call the change done: The link is named. The length is written. Both addresses are written. No router was changed. And a second person agrees. If one of those fails, stop.
| Length | What you are accepting |
|---|---|
| /30 | Two usable addresses, plus network and broadcast. |
| /31 | Both addresses are hosts. Only for a link of two routers. |
| A LAN | Neither. A LAN needs room for hosts. |
Before you leave the device
/30 vs /31 for Point-to-Point Links 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
These tools run in the browser on this site. They do not log into your machines.
Related pillar guides
More in this cluster
The pillar for this page, and the next published pages filed with it.