Subnetting helps split a big network into smaller, manageable parts called subnets. 

It’s like carving up a neighborhood into multiple streets so traffic flows better instead of jamming up one road. 

It helps:

• Organize networks into smaller sections
• Reduce wasted IP addresses
• Improve security and performance

Subnetting is essential for:

• Network engineers
• Cybersecurity professionals
• Developers working with networked apps
• Anyone studying for networking certifications like CompTIA Network+, CCNA, etc.

Every IPv4 address (like 192.168.0.1) is made of:

• Network bits — define which network you’re in
• Host bits — define individual devices on that network

A subnet mask tells your computer where the “network” part ends and the “host” part begins.

Example:

• IP Address: 192.168.0.101
• Subnet Mask: 255.255.255.0
• Network ID: 192.168.0.0
• Host part: .101

Here’s a simplified table for quick reference:

For the full cheat sheet, visit:
FreeCodeCamp Subnet Cheat Sheet

CIDR = Classless Inter-Domain Routing.

Instead of traditional “Class A, B, C” networks, CIDR lets engineers:

• Split networks into subnets of different sizes (Variable Length Subnet Masking, or VLSM)
• Avoid wasting IP addresses

CIDR notation uses a slash, e.g. /24, to show how many bits belong to the network portion.

Example:

• 192.168.0.0/24
  → subnet mask = 255.255.255.0

How to Calculate CIDR Notation

1. Convert subnet mask to binary.
   • e.g. 255.255.255.0 = 11111111.11111111.11111111.00000000
2. Count the number of “1” bits.
   • In this case → 24 bits → so CIDR = /24

• Subnet Mask: Defines network and host bits.
• Wildcard Mask: Inverse of subnet mask, used in some configurations.

Example:

Subnet Mask: 255.255.255.0

Wildcard Mask: 0.0.0.255

Before CIDR, networks were divided into:

• Class A: 1.0.0.0 – 127.255.255.255 (/8)
• Class B: 128.0.0.0 – 191.255.255.255 (/16)
• Class C: 192.0.0.0 – 223.255.255.255 (/24)

Problem: Wasted addresses. For example:

• Class B = 65,534 usable hosts
• Many businesses only needed a few thousand.

CIDR fixed this by letting us customize subnet sizes to fit our needs.

• Can’t tell if two IPs are on the same network?
  • Compare the network bits using the subnet mask.
• Network connectivity issues?
  • Check if devices are on the same subnet.
• Confused by CIDR?
  • Remember:
    • /24 → 255.255.255.0
    • /16 → 255.255.0.0
    • /8 → 255.0.0.0

•     FreeCodeCamp Subnet Cheat Sheet
•     Cisco Subnetting Reference Chart
•     Packet Coders Guide
 

Click here to schedule a 1:1 with a tutor, coach, and or sign up for a workshop. *If this link does not bring you directly to our platform, please use our direct link to "Academic Support" from any Brightspace course at the top of the navigation bar. 

This resource was organized using ChatGPT.

 OpenAI. (2023). ChatGPT (Mar 14 version) [Large language model]. https://chat.openai.com/chat