Cheatography
https://cheatography.com
Intro
The 30-second subnetting trick, CIDR tables, and everything CompTIA and Cisco expect you to know cold. |
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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 |
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
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More from ByteBar
Full subnetting walkthroughs + practice questions at bytebarhq.com — by ByteBar. |
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