Subnetting is one of the most important skills in networking. It's how large networks are divided into smaller, organized pieces. Think of it like organizing a big office building into separate departments-each department has its own space, resources, and security.

In this room, you'll learn the fundamentals of subnetting in a simple, step-by-step way. We'll start with what you already know about IP addresses and build up to planning a network for a small business. While subnetting can seem challenging at first, with clear explanations and practice, you'll understand how it works.

Why Subnetting Matters

Subnetting isn't just theory-it's used in every network:

• Efficiency: Prevents IP address waste by creating right-sized networks
• Performance: Reduces network congestion by limiting broadcast traffic
• Security: Allows you to isolate network segments from each other
• Organization: Makes large networks easier to manage and troubleshoot

What You'll Learn

By the end of this room, you will understand:

• How IP addresses are structured for subnetting
• What subnet masks are and how to read them
• Modern CIDR notation (like /24 or /26)
• How to calculate the number of hosts in a subnet
• How to apply subnetting to a real-world scenario

Prerequisites

Before starting, you should already be familiar with:

• Basic IP addressing (IPv4 format)
• The difference between network and host
• Simple binary concepts (bits and bytes)
• Basic networking terminology

How to Approach This Room

1. Take it slow: Subnetting builds on each concept
2. Practice the examples: Try calculations yourself
3. Use the visualizations: They help make abstract concepts concrete
4. Don't worry about memorizing: Focus on understanding the principles

Optional Video

This optional video covers the fundamental concepts of subnetting. It's helpful but not required to complete the room.

IP Address Format

Every device on a network needs a unique identifier, an IP address. You've already learned about IPv4 addresses, which look like four numbers separated by dots: 192.168.1.10. Each number is called an octet because it represents 8 bits in binary.

The Two Parts of Every IP Address

For subnetting, the most important concept is that every IP address has two parts:

• Network portion: Identifies which network the device is on
• Host portion: Identifies the specific device on that network

Think of it like a street address:

• Street name = Network portion (tells you which neighborhood)
• House number = Host portion (tells you which specific house)

Binary Basics for Subnetting

Subnetting works with the binary form of IP addresses. Don't worry, we'll keep it simple! Computers see 192.168.1.10 as 32 ones and zeros:

Decimal: 192 . 168 . 1 . 10
Binary: 11000000 . 10101000 . 00000001 . 00001010

The key insight: The subnet mask will tell us how many of these 32 bits are for the network vs. the host.

Below is a visual showing the IP address 192.168.1.10 split into its 4 octets, with the binary equivalent (11000000.10101000.00000001.00001010) shown below each. The 32-bit sequence should be visually grouped into four 8-bit blocks.

Quick Binary Reference

Here's how the first octet converts (you don't need to memorize this, just understand the pattern):

Decimal	Binary	Decimal	Binary
0	00000000	128	10000000
64	01000000	192	11000000
96	01100000	224	11100000
112	01110000	240	11110000
120	01111000	248	11111000
124	01111100	252	11111100
126	01111110	254	11111110
127	01111111	255	11111111

Key Takeaway

Remember: All subnetting decisions are based on separating the network bits from the host bits in the 32-bit IP address. The next task will show you exactly how we make that separation.

A subnet mask is a 32-bit number that tells devices on a network which part of an IP address is the network portion and which part is the host portion. It acts like a template or filter that reveals only the network information.

Like an IP address, a subnet mask is written in dotted-decimal format. A very common example is 255.255.255.0.

How It Works: The 1s and 0s Rule

The rule is simple:

• Bits set to 1 (255 in decimal) identify the network portion.
• Bits set to 0 (0 in decimal) identify the host portion.

For the mask 255.255.255.0:

• The first three octets (255.255.255) are all 1s in binary - they cover the network bits.
• The last octet (0) is all 0s in binary, it leaves room for host addresses.

Reading a Subnet Mask with an IP Address

Let's pair an IP address with its subnet mask:

• IP Address: 192.168.1.10
• Subnet Mask: 255.255.255.0

This combination tells every device: "The network is 192.168.1.x, and 10 is a specific host on that network." Any device with an IP starting with 192.168.1 is on the same local network.

Visual Demonstration: Subnet Mask Overlay

Below is a side-by-side or overlay visual comparing the IP address 192.168.1.10 and the subnet mask 255.255.255.0. Show their binary forms aligned. Visually highlight or color the network bits (the first 24 bits covered by 255.255.255) in one color and the host bits (the last 8 bits covered by .0) in another color.

Common Default Subnet Masks

In older classful networking, certain masks were standard. You'll still see these often:

Network Class	Default Subnet Mask	Typical Use
Class A	255.0.0.0	Very large networks
Class B	255.255.0.0	Medium-sized organizations
Class C	255.255.255.0	Small offices or home networks

The "AND" Operation

Your computer performs a logical AND operation between the IP address and the subnet mask to determine the network address. Don't worry about the math for now-just understand that the mask filters out the host bits, leaving only the network identity.

Key Takeaway

A subnet mask is the key that unlocks how an IP address is interpreted on a network. It defines the boundary between "where you are" (the network) and "who you are" (the host) on that network.

What is CIDR Notation?

CIDR (Classless Inter-Domain Routing) notation is the modern, shorthand way to represent a subnet mask. Instead of writing out all four octets like 255.255.255.0, you simply write a slash followed by a number, like /24.

Understanding the Slash Number

The number after the slash (/24) tells you how many bits in the IP address are used for the network portion. It counts the consecutive 1s from left to right in the subnet mask's binary form.

Example:

• Subnet Mask: 255.255.255.0
• Binary: 11111111.11111111.11111111.00000000
• Count the 1s: 8 + 8 + 8 = 24
• CIDR Notation: /24

Below is a visual flowchart illustrating how a /24 CIDR block determines the number of host bits, total addresses, reserved network and broadcast addresses, and the resulting number of usable hosts.

Common CIDR Values and Their Masks

This table shows the most common CIDR notations you will encounter:

CIDR Notation	Subnet Mask	Network Bits	Host Bits
/8	255.0.0.0	8	24
/16	255.255.0.0	16	16
/24	255.255.255.0	24	8
/26	255.255.255.192	26	6
/28	255.255.255.240	28	4
/30	255.255.255.252	30	2

Why CIDR Replaced Classful Networking

Classful networking (Class A, B, C) was very rigid and wasted many IP addresses. CIDR is classless, meaning you can create subnets of almost any size by simply changing the number of network bits. This flexibility saves addresses and allows for more efficient network design.

Try It Yourself

What is the CIDR notation for a subnet mask of 255.255.0.0?

1. Binary: 11111111.11111111.00000000.00000000
2. Count the 1s: 8 + 8 = 16
3. Answer: /16

Key Takeaway

CIDR notation (/24) is simply a compact way to write a subnet mask. The number represents the count of network bits. It gives you immediate insight into how large or small a network is.

The Core Question: How Many Hosts?

Once you know a subnet's CIDR notation (like /24), the most practical question is: How many devices can I connect to this network? To find the answer, we need to calculate the number of usable host addresses.

The Formula

The calculation follows a clear, two-step formula:

1. Find the number of host bits: 32 - [CIDR number] = Host Bits
   Example for /24: 32 - 24 = 8 host bits.
2. Calculate usable hosts: 2^(Host Bits) - 2 = Usable Hosts
   Example with 8 host bits: 2^8 - 2 = 256 - 2 = 254 usable hosts.

Why Minus 2?

Every subnet reserves two addresses that cannot be assigned to regular devices:

• Network Address: The first address (all host bits set to 0). It identifies the network itself.
• Broadcast Address: The last address (all host bits set to 1). It is used to send data to all devices on the subnet.

Below is a step-by-step flowchart for calculating usable hosts. Start with CIDR /24, subtract from 32 to get "8 Host Bits", show the calculation 2^8 = 256, then subtract 2 to get the final answer: "254 Usable Hosts". Visually set aside the "First Address (Network)" and "Last Address (Broadcast)" to illustrate the "-2".

Walkthrough Example: A /26 Network

Let's calculate for a /26 network, which is common for smaller segments.

1. Host Bits: 32 - 26 = 6 host bits.
2. Total Addresses: 2^6 = 64 total addresses in the subnet.
3. Usable Hosts: 64 - 2 = 62 devices that can be assigned IPs.

Quick Reference Table

Here is a handy reference for some common subnet sizes:

CIDR	Subnet Mask	Host Bits	Total Addresses	Usable Hosts
/30	255.255.255.252	2	4	2
/29	255.255.255.248	3	8	6
/28	255.255.255.240	4	16	14
/27	255.255.255.224	5	32	30
/26	255.255.255.192	6	64	62
/25	255.255.255.128	7	128	126
/24	255.255.255.0	8	256	254

Practice Scenario

A small office needs a network for about 30 employees. Looking at the table, which CIDR value would provide enough usable hosts without too much waste?

• /27 gives 30 usable hosts-perfect for about 30 devices.
• /26 would give 62 hosts, but that's more than needed and would waste addresses.

Choosing the right subnet size is about balancing your needs with efficient use of IP addresses.

Key Takeaway

The power of subnetting is controlling size. With the 2^(host bits) - 2 formula, you can plan exactly how large to make each network segment, from a huge /16 network to a tiny /30 link between two routers.

Applying Your Knowledge: A Small Business Network

You are the network administrator for "QuickTech Solutions," a growing small business. You need to design the IP addressing scheme for their new office. The office has three distinct groups that need to be on separate network segments for security and management:

• Sales Team: 12 employees with computers and phones
• Administration: 8 employees with computers
• Guest WiFi: For visitors, expecting up to 25 devices at once

Your task is to subnet the company's assigned network block, 192.168.10.0/24, to meet these needs.

Scenario Requirements

Department	Devices Needed	Key Requirement
Sales	12	Room for some growth
Admin	8	Basic connectivity
Guest WiFi	25	Must accommodate peak visitor count

Goal: Create three separate subnets from 192.168.10.0/24. Each subnet must have enough usable host addresses for its department.

Planning the Subnets

Let's break this down using what we've learned.

1. Determine Required Subnet Size:
   • Sales (12 devices): We need a subnet with at least 12 usable hosts. Looking at our reference table, a /28 subnet (14 usable hosts) is the smallest that fits.
   • Admin (8 devices): A /28 subnet (14 usable hosts) also works perfectly here.
   • Guest (25 devices): We need at least 25 usable hosts. A /27 subnet (30 usable hosts) is the right choice.
2. Choose CIDR and Calculate Details:
   We'll allocate the larger subnet to Guest WiFi first, then the two smaller, equal ones to Sales and Admin.

Below is a simple network diagram showing one router connected to three separate network clouds or switches, labeled "Guest WiFi (/27)", "Sales (/28)", and "Admin (/28)". Inside each cloud, list the Network Address and Usable Host Range from the table above. Visually show that all three subnets come from the original 192.168.10.0/24 block.

Proposed Subnet Plan

Based on our calculations, here is a workable solution:

Department	Network Address	CIDR	Subnet Mask	Usable Host Range	Broadcast Address	Usable Hosts
Guest WiFi	192.168.10.0	/27	255.255.255.224	192.168.10.1 - 192.168.10.30	192.168.10.31	30
Sales	192.168.10.32	/28	255.255.255.240	192.168.10.33 - 192.168.10.46	192.168.10.47	14
Admin	192.168.10.48	/28	255.255.255.240	192.168.10.49 - 192.168.10.62	192.168.10.63	14

The remaining addresses (192.168.10.64 to 192.168.10.255) are left unused in the original /24 block for future expansion.

Verification and Notes

• Check: Each department's "Usable Hosts" count meets or exceeds their "Devices Needed."
• Separation: Devices in one department cannot directly talk to devices in another without going through a router (which is good for security!).
• Real-World Note: In an actual setup, you would configure a router or layer-3 switch with these three subnet addresses and set up firewall rules between them. For example, you would likely block the Guest WiFi from accessing the Sales and Admin subnets entirely.

Congratulations! You've tackled a topic that many find challenging and have built a solid foundation for understanding how networks are organized and segmented.

What You've Learned

Let's review the key concepts you now understand:

• IP Structure: Every IPv4 address is a 32-bit number divided into a network portion and a host portion.
• Subnet Mask: A 32-bit filter (like 255.255.255.0) that defines the boundary between the network and host bits in an IP address.
• CIDR Notation: The modern, shorthand way to represent a subnet mask by counting its network bits (e.g., /24).
• Host Calculation: The formula to find usable devices in a subnet: 2^(Host Bits) - 2, where Host Bits = 32 - CIDR number.
• Practical Application: How to apply subnetting to divide a single network block (like 192.168.10.0/24) into multiple smaller, efficient subnets for different purposes.

You Should Now Be Able To...

• Look at an IP address and subnet mask (or CIDR) and identify the network it belongs to.
• Determine how many usable host addresses are available in a given subnet.
• Understand why a network administrator would create multiple subnets.
• Read and interpret basic network diagrams that include CIDR notation.