Subnetting

September 12, 2025 note-to-self full-stack

Subnetting Cheat Sheet**:

128 64  32  16  8   4   2   1
128 192 224 240 248 252 254 255
/25 /26 /27 /28 /29 /30 /31 /32
/17 /18 /19 /20 /21 /22 /23 /24
/9  /10 /11 /12 /13 /14 /15 /16
/1  /2  /3  /4  /5  /6  /7  /8

Using that cheatsheet, solve for:

Network ID:
First Usable IP:
Last Usable IP:
Broadcast ID:
Next Network:
Subnet Mask:
Number of Hosts:

Subnetting practice: https://subnetipv4.com/

With subnet mask practice: https://richtechguy.com/subnetting-practice-tool/

CIDR Calculator: https://cidr.xyz/

Networking Classes

Class	1st Octet Range	Subnet Mask	Description
A	1–126	255.0.0.0	Designed for a small number of networks with a very large number of hosts on each one.
B	128–191	255.255.0.0	Used for medium-sized networks.
C	192–223	255.255.255.0	Used for a large number of networks with a small number of hosts on each one.
D	224–239	N/A	Reserved for multicast traffic.
E	240–255	N/A	Reserved for experimental use.

Useful Linux app:

ipcalc --split 2 $(curl -s ha17.com/ip)

Might need:

sudo apt update && sudo apt install ipcalc

Private Network Ranges

10.0.0.0 to 10.255.255.255 (Class A): This range offers over 16 million IP addresses and is typically used for large networks, like large corporations.
172.16.0.0 to 172.31.255.255 (Class B): This range provides more than one million IP addresses and is suitable for medium-sized networks, such as schools or medium-sized businesses.
192.168.0.0 to 192.168.255.255 (Class C): This range provides over 65,000 IP addresses and is commonly used for home and small office networks.

** I needed this a lot at first but suddenly find it pretty easy to do this from memory.

Most posts are for my own reference and reflection, and shouldn’t be taken as fully accurate or instructional.