Subnetting Cheat Sheet

Quick table

Common prefixes from /24 through /30

CIDR	Mask	Block size	Total addresses	Usable hosts*
/24	255.255.255.0	256	256	254
/25	255.255.255.128	128	128	126
/26	255.255.255.192	64	64	62
/27	255.255.255.224	32	32	30
/28	255.255.255.240	16	16	14
/29	255.255.255.248	8	8	6
/30	255.255.255.252	4	4	2

Swipe horizontally to see the full table.

*Traditional IPv4 LAN host counts subtract the network and broadcast addresses.

Mask values

Partial-octet values to memorize once

Network bits in octet	Binary	Decimal
1	10000000	128
2	11000000	192
3	11100000	224
4	11110000	240
5	11111000	248
6	11111100	252
7	11111110	254

Swipe horizontally to see the full table.

These seven values explain nearly every common dotted-decimal mask conversion. A full eight network bits makes 255.

Boundary shortcut

Find the block size with 256 minus the mask value

For 255.255.255.224, calculate 256 − 224 = 32. Subnet boundaries in that octet occur at 0, 32, 64, 96, 128, 160, 192, and 224. For 255.255.255.240, the block size is 16. This shortcut is fast because it turns a binary boundary into a simple decimal interval.

Four-step method

The shortest reliable subnetting workflow

1. Convert the prefix to a mask or identify the interesting octet.
2. Find the block size with 256 minus the interesting mask octet.
3. Locate the interval containing the host address.
4. Label the boundaries: first address = network, last address before next block = broadcast, addresses between = traditional host range.

Example /27

192.168.44.173/27

The /27 mask is 255.255.255.224, block size 32. Boundaries are .0, .32, .64, .96, .128, .160, .192, and .224. Address .173 lies in the .160–.191 block. Network = .160, broadcast = .191, usable hosts = .161–.190, 30 traditional host addresses.

Example /28

10.5.7.94/28

★ Addressing Practice • Subnetting Cheat Sheet

A five-minute warm-up for Subnetting Cheat Sheet

Tomorrow’s opening question can pick up where this page leaves off: have students explain where the addresses come from and what they would check when a device cannot connect. The 300 Bell Ringers give you short questions and teacher answers you can use without preparing a new starter each morning.

300 AP Networking Bell Ringers

$74.99

BUY TEACHER PACK → See what is included in the 300 Bell Ringers →

Instant Access • Full-Year Teacher Resource

The /28 mask is 255.255.255.240, block size 16. Boundaries include .80 and .96, so .94 lies in 10.5.7.80/28. Broadcast is .95; usable range is .81–.94. Notice that the given host .94 is itself the final usable host, not the broadcast.

Addressing & Subnetting Lab • 120 Missions

Let students try Subnetting Cheat Sheet for themselves

Reading the explanation is a start. In the browser simulator, students work through a broken network, choose what to test and see whether their fix worked. It gives them another chance to use what they learned about Subnetting Cheat Sheet.

AP Networking Browser Troubleshooting Simulator

$29.99

120 Missions • 60 Failure Patterns • Offline Copy

GET THE SIMULATOR → See all 120 missions & features

Same subnet

Fast same-subnet check

Addressing Exam Practice • Subnetting Cheat Sheet

Check whether Subnetting Cheat Sheet has really stuck

Once students have worked through this topic, give them questions mixed in with the rest of the course. The Exam Prep Vault includes practice exams, multiple-choice questions and artifact labs so they cannot rely on a chapter heading to tell them what to do.

AP Networking Complete Exam Prep Vault

$39.99

GET THE EXAM PREP VAULT → See the Exam Prep Vault contents →

4 Practice Exams • 320 MCQs • 60 Artifact Labs

Calculate the network address for both hosts with the stated prefix. If the network addresses match, the hosts are in the same subnet. Example: .63/26 belongs to the .0 block; .64/26 belongs to the .64 block. Adjacent decimal values can still be on opposite sides of a subnet boundary.

Premium Routing Feedback

Spend less time marking explanations about Subnetting Cheat Sheet

When students explain their answer, it takes more than a right-or-wrong check to see what they understand. The Grading Copilot helps you review how they used the clues, where their explanation falls short and what feedback to give next. You make the final grading decision.

AP Networking Artifact Analysis Grading Copilot

$49.99

8 Criteria • 5 Task Modes • Offline Premium Copy

GET THE GRADING COPILOT → See the free Copilot preview

Do not memorize blindly

The cheat sheet should support reasoning, not replace it

NEW • COMPLETE 180-DAY TEACHER CURRICULUM

Teach Subnetting Cheat Sheet as part of a planned school year

This page gives you one topic. The 180-Day Teacher Curriculum puts it into a full course plan, with daily teaching notes, student activities and checks for understanding. Use it when you need to know what comes before this lesson and what to teach next.

AP Networking Complete 180-Day Teacher Curriculum

$199.99

180 Lessons • 180 Daily Simulators • 36 Teaching Weeks

GET THE 180-DAY CURRICULUM → See 3 free curriculum examples

If you can explain why /27 produces 32-address blocks and why the first and last addresses have special roles, the table becomes a speed aid. If you only memorize “/27 = 30 hosts,” a changed question about broadcast address or same-subnet behavior can still cause trouble.

Use Subnetting Practice Questions after this reference page.