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
CIDR is a prefix length on an address. 192.0.2.0/24 means the first 24 bits are the network. Two addresses are in the same subnet when those bits match. The mask 255.255.255.0 is the same statement written another way.
The converters draft a range from a prefix or a prefix from a range. They do not configure a router. You still check that the mask and the prefix describe one boundary. A person who types the wildcard into a host mask field builds a different subnet.
Private space is 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Documentation ranges are 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32. Do not paste a documentation range into production because it was in the example.
Class A, B, and C are not how you design. A /24 is not a class. Write the prefix and the mask in the ticket together.
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.
- Write the prefix you mean.
- Convert it and read the first and last address.
- Write the mask beside the prefix.
- Reject a documentation range for production.
- Hand the plan to the subnetting how-to if you must carve it.
- Stop after one prefix.
Mistakes that outlive the command
CIDR Notation Explained goes wrong in a small set of ways. The list is here so the ticket can forbid them before the window.
- A class used instead of a prefix length.
- The wildcard typed as the mask.
- A documentation range applied in production.
- Two prefixes described as one.
- Trusting the converter without reading the ends.
Decisions to write down
One decision for CIDR Notation Explained: The prefix length is the boundary. The mask matches that length. The range ends were read. Documentation ranges stayed in the note. One prefix is this window. And the subnetting page owns the carving.
Checks before you call it done
Confirm this before you call the change done: The prefix and the mask are both written. The first and last addresses match the length. No documentation range was applied. A second person can say the boundary. And the converter was not treated as a push to the router. If one of those fails, stop.
192.0.2.0/24 is documentation.
Read the mask before you type it.Before you leave the device
CIDR Notation 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
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.
- Pillar: Subnetting Explained Step by Step
- Cheat Sheet: Subnet Mask Cheat Sheet
- How-To: How to Subnet a Network in 5 Steps
- Explainer: VLSM Explained with Examples
Companion: Cloud: VPC CIDR Planner.