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Subnetting Notes

Overview

Subnetting is the process of dividing a larger network into smaller networks. It helps organize devices, reduce broadcast traffic, improve security, and manage IP address usage.

Subnetting is an important networking skill for help desk, IT support, junior network administrator, and CCNA-level study.

Purpose

This document explains:

  • What subnetting is
  • Why subnetting is used
  • Common subnet masks
  • CIDR notation
  • Network and host addresses
  • Basic subnetting examples

Key Terms

Term Meaning
IP Address Identifies a device on a network
Subnet Mask Separates network portion and host portion
CIDR Slash notation that shows network bits
Network Address First address in a subnet
Broadcast Address Last address in a subnet
Usable Host Range Addresses assigned to devices
Default Gateway Router address used to leave the subnet

Example IP Configuration

IP Address: 192.168.1.25
Subnet Mask: 255.255.255.0
CIDR: /24
Default Gateway: 192.168.1.1

This means the device is on the 192.168.1.0/24 network.

CIDR Notation

CIDR uses a slash to show how many bits are used for the network.

Example:

192.168.1.0/24

The /24 means 24 bits are used for the network portion.

Common Subnet Masks

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

Two addresses are usually reserved:

  • Network address
  • Broadcast address

Example: /24 Network

Network:

192.168.1.0/24

Details:

Item Value
Network Address 192.168.1.0
First Usable Host 192.168.1.1
Last Usable Host 192.168.1.254
Broadcast Address 192.168.1.255
Usable Hosts 254

Example: /25 Network

A /25 splits a /24 into two subnets.

Subnet 1:

192.168.1.0/25
Item Value
Network Address 192.168.1.0
First Usable Host 192.168.1.1
Last Usable Host 192.168.1.126
Broadcast Address 192.168.1.127

Subnet 2:

192.168.1.128/25
Item Value
Network Address 192.168.1.128
First Usable Host 192.168.1.129
Last Usable Host 192.168.1.254
Broadcast Address 192.168.1.255

Example: /26 Network

A /26 creates blocks of 64 addresses.

Subnets in 192.168.1.0/24:

192.168.1.0/26
192.168.1.64/26
192.168.1.128/26
192.168.1.192/26

First subnet:

Item Value
Network Address 192.168.1.0
First Usable Host 192.168.1.1
Last Usable Host 192.168.1.62
Broadcast Address 192.168.1.63
Usable Hosts 62

Quick Block Size Method

To find subnet block size:

Block size = 256 - subnet mask value

Example:

/26 = 255.255.255.192
256 - 192 = 64

So the subnets increase by 64:

0, 64, 128, 192

Why Subnetting Is Used

Subnetting helps with:

  • Separating departments
  • Improving network organization
  • Reducing broadcast traffic
  • Supporting VLAN design
  • Improving security
  • Managing IP address usage
  • Supporting routing between networks

Example business layout:

Department Subnet
HR 192.168.10.0/24
Sales 192.168.20.0/24
IT 192.168.30.0/24
Guest Wi-Fi 192.168.40.0/24

Troubleshooting With Subnets

If two devices cannot communicate, check:

  • Are they in the same subnet?
  • Do they have correct subnet masks?
  • Do they have the correct default gateway?
  • Is routing configured between subnets?
  • Is a firewall blocking traffic?
  • Are they on the correct VLAN?

Example issue:

Device A: 192.168.1.25 /24
Device B: 192.168.2.25 /24

These devices are on different subnets. They need routing through a gateway to communicate.

Practice Questions

Question 1

Network:

192.168.5.0/24

How many usable hosts?

Answer:

254 usable hosts

Question 2

Network:

192.168.10.0/26

What is the block size?

Answer:

256 - 192 = 64

Question 3

Subnets for 192.168.10.0/26:

Answer:

192.168.10.0
192.168.10.64
192.168.10.128
192.168.10.192

Skills Demonstrated

  • Subnet mask understanding
  • CIDR notation
  • IP address planning
  • Network and broadcast address identification
  • Host range calculation
  • VLAN and routing awareness
  • Networking fundamentals