Cheatography
Login Register

Subnetting Cheat Sheet

A cheat sheet in English by bytebar
591 views 192 downloads 14 Sep 2026 History
PRs
Download

Intro

The 30-second subnetting trick, CIDR tables, and everything CompTIA and Cisco expect you to know cold.
Intro text

The Subnetting Trick (do this on every question)

Given an IP and CIDR (e.g. 192.16­8.1.77­/26):
1. Block size = 256 − intere­sting octet value → 256 − 192 = 64
2. Subnets fall on multiples of the block size → 0, 64, 128, 192
3. 77 falls between 64 and 128 → subnet is 192.16­8.1.64
4. Range: 192.16­8.1.64 – 192.16­8.1.127 · Broadcast: .127 · Usable: .65 – .126
Subnetting trick steps

CIDR Quick Table

CIDR
Subnet Mask
Wildcard
Usable Hosts
/24
255.25­5.255.0
0.0.0.255
254
/25
255.25­5.2­55.128
0.0.0.127
126
/26
255.25­5.2­55.192
0.0.0.63
62
/27
255.25­5.2­55.224
0.0.0.31
30
/28
255.25­5.2­55.240
0.0.0.15
14
/29
255.25­5.2­55.248
0.0.0.7
6
/30
255.25­5.2­55.252
0.0.0.3
2
/23
255.25­5.254.0
0.0.1.255
510
/22
255.25­5.252.0
0.0.3.255
1,022
/21
255.25­5.248.0
0.0.7.255
2,046
/16
255.25­5.0.0
0.0.25­5.255
65,534
Formula: usable hosts = 2^(host bits) − 2 (subtract network + broadcast)

Private IP Ranges (RFC 1918)

Class
Range
CIDR
Addresses
A
10.0.0.0 – 10.255.25­5.255
/8
16,777,216
B
172.16.0.0 – 172.31.25­5.255
/12
1,048,576
C
192.16­8.0.0 – 192.16­8.2­55.255
/16
65,536
Also know: 127.0.0.0/8 = loopback · 169.25­4.0.0/16 = APIPA (link-­local, DHCP failed)

Classful Boundaries

Class
First Octet
Default Mask
A
1 – 126
/8
B
128 – 191
/16
C
192 – 223
/24
(127 is reserved — loopback, not Class A usable.)

Specia­l-P­urpose Addresses

Address
Meaning
0.0.0.0/0
Default route — "­eve­ryt­hin­g"
255.25­5.2­55.255
Limited broadcast — "­eve­ryone here"
Network address
First address in subnet (not assign­able)
Broadcast address
Last address in subnet (not assign­able)
Special addresses to know

More from ByteBar

Full subnetting walkth­roughs + practice questions at byteba­rhq.com — by ByteBar.
📘 Exam-r­eady? Get the Subnetting Mastery Pack — practice questions with full explan­ations, $7 at byteba­rhq.com
Footer

Master Subnetting — Take It Further

Like this cheat sheet? The Subnetting Mastery Pack gives you the full ByteBar subnetting guide plus 30 practice questions with complete explan­ations — only $3: https:­//b­yte­bar­hq.c­om­/b/­BN0PV
Footer upsell block

Comments

No comments yet.

Cheatography n. The art of mapping complex subjects into clear, instantly navigable reference guides.