Learning Objectives

• Understand why subnetting exists
• Learn the difference between classful and classless addressing
• See how subnetting splits a network into smaller pieces

Why Subnetting?

Imagine a large office building with one giant room. Everyone shares the same space — conversations overlap, there's no privacy, and finding the right person is difficult. Now imagine that same building divided into separate rooms and floors. Each room has a purpose, people can work without interference, and you know exactly where to find someone.

That's subnetting.

A subnet (short for sub-network) is a logical subdivision of an IP network. Instead of having one large network where every device can talk to every other device directly, we break it into smaller, manageable pieces.

Before Subnetting: Classful Addressing

Originally, IP addresses were divided into classes:

| Class | Range | Default Mask | Networks | Hosts per Network | |-------|-------|-------------|----------|-------------------| | A | 1.0.0.0 - 126.255.255.255 | /8 (255.0.0.0) | 126 | 16,777,214 | | B | 128.0.0.0 - 191.255.255.255 | /16 (255.255.0.0) | 16,384 | 65,534 | | C | 192.0.0.0 - 223.255.255.255 | /24 (255.255.255.0) | 2,097,152 | 254 |

This was wasteful. If you needed 200 addresses, you got a Class C (254 usable). If you needed 300, you had to take a Class B (65,534 usable) — wasting over 65,000 addresses.

Enter CIDR: Classless Inter-Domain Routing

CIDR (pronounced "cider") replaced the rigid class system with a flexible prefix-length notation. Instead of being locked into /8, /16, or /24, you can use any prefix length from /0 to /32.

This is where subnetting gets powerful. You can take a /24 network like 192.168.1.0/24 and split it into:

• 2 × /25 subnets (128 addresses each)
• 4 × /26 subnets (64 addresses each)
• 8 × /27 subnets (32 addresses each)
• 16 × /28 subnets (16 addresses each)
• 32 × /29 subnets (8 addresses each)
• 64 × /30 subnets (4 addresses each)

Why It Matters Today

Every modern network uses subnetting:

• Home networks — Your router creates a /24 subnet (typically 192.168.1.0/24)
• Enterprise networks — Hundreds of subnets segmented by department, security zone, or function
• Cloud VPCs — AWS, Azure, and GCP all require you to design subnets for your virtual networks
• Data centers — Subnets isolate traffic between tiers (web, app, database)

Key Takeaways

• Subnetting divides a large network into smaller, efficient pieces
• CIDR replaced the wasteful classful system
• Every network, from your home Wi-Fi to global data centers, uses subnetting
• The prefix length (the /xx in CIDR notation) is the key to understanding subnetting

In the next lesson, we'll dive into CIDR notation and how prefix lengths work.