CIDR Challenge

An IPv4 subnetting trainer for the Flipper Zero.

CIDR Challenge is not a subnet calculator. A calculator hands you the answer; this app makes you find it. Every exercise is generated on the fly, the wrong options are built from the mistakes people actually make, and the whole thing runs offline on the device.

10 minutes a day to become faster at networking.

Question types

Question	Example	Answer
Network address	10.10.15.130/26	10.10.15.128
Broadcast address	172.16.20.64/28	172.16.20.79
First usable host	192.168.1.0/24	192.168.1.1
Last usable host	192.168.1.0/24	192.168.1.254
Available hosts	10.0.0.0/22	1022
Mask to CIDR	255.255.255.224	/27
CIDR for a host count	62 hosts needed	/26

Each exercise offers four choices. The three distractors come from classic subnetting errors: forgetting the -2, using the neighbouring prefix, mixing up the network and broadcast address, or snapping to the wrong block boundary.

Game modes

Training — free practice, no pressure, run it as long as you like. Every answer is scored right away and the correct option is highlighted, so a wrong guess still teaches you something.

Speed Challenge — 60 seconds on the clock. Every correct answer adds to the score, the timer never stops.

Survival — a single mistake ends the run. The header fills with streak pips as you go.

Exam — certification-style simulation over 10, 25 or 50 questions with mixed masks, closed by a scored report.

Difficulty

Level	Prefixes
Beginner	/24 – /26
Intermediate	/20 – /30
Advanced	/8 – /32, including the /31 and /32 edge cases

Advanced follows RFC 3021 for point-to-point links: a /31 has 2 usable addresses and a /32 has 1. Everything from /8 to /30 uses the classic 2^n - 2 rule.

Progression

Reward	XP
Correct answer	+10
Answered in under 5 s	+5
Every 5 answers in a row	+25

100 XP per level, up to level 30.

Level	Title
1	Subnet Beginner
3	Network Apprentice
5	Network Student
8	Subnet Engineer
10	CIDR Master
15	Packet Guru
20	Routing Wizard

Lifetime statistics — questions, correct answers, accuracy, best streak and fastest answer — are kept on the SD card. Hold OK on the Statistics screen to wipe them.

Controls

Key	Action
Up / Down / Left / Right	Move between answers and menu entries
OK	Confirm, and continue after the feedback
Back	Leave the current screen, end the run

Storage

Everything is written to /ext/apps_data/cidr_challenge/:

• stats.txt — XP, questions, correct answers, best streak, fastest answer
• settings.txt — difficulty, exam length, sound

Both files are plain Flipper format text. If they are missing or damaged the app starts from clean defaults, so deleting them is a valid reset.

Install

Copy dist/cidr_challenge.fap to /ext/apps/Games/ on the SD card, either over qFlipper or by putting the card in a reader. The app shows up under Apps → Games → CIDR Challenge.

Build

The app is a standard external FAP with no dependencies beyond the SDK.

With the official firmware:

git clone https://github.com/flipperdevices/flipperzero-firmware.git
git clone https://github.com/AlastorApps/cidrchallengef0.git \
    flipperzero-firmware/applications_user/cidr_challenge
cd flipperzero-firmware
./fbt fap_cidr_challenge

The result lands in build/f7-firmware-D/.extapps/cidr_challenge.fap. Add COMPACT=1 DEBUG=0 for a release build, or use ./fbt launch APPSRC=cidr_challenge to build and run it straight on a connected Flipper.

With ufbt, from a clone of this repository:

ufbt
ufbt launch

Layout

application.fam        app manifest
cidr_challenge.c/.h    app lifecycle, scenes, input handling
quiz.c/.h              exercise generation and distractors
subnet.c/.h            IPv4 subnet math
progress.c/.h          XP, levels, statistics, settings, storage
ui.c                   all screen drawing
icons/                 application icon
screenshots/           framebuffer captures used in this README
dist/                  prebuilt cidr_challenge.fap

Compatibility

Tested on:

• Flipper Zero
• Official firmware
• Momentum firmware
• RogueMaster firmware

Requires:

• Internal storage only
• No external modules
• No internet connection
• No additional databases

Contributing

Pull requests and suggestions are welcome.

Ideas, bug reports and feature requests can be opened through GitHub Issues.

Who is this for?

• Network+ students
• CCNA candidates
• Cybersecurity students
• Pentesters
• Network administrators
• Anyone wanting to improve IPv4 subnetting speed

License

GPL-3.0. See LICENSE.

Created by Alastor — https://github.com/AlastorApps/cidrchallengef0