IPv4 Addressing & Subnetting

Learn subnet concepts including CIDR (slash notation), subnet masks, broadcast domains, and how subnet calculators work.

IPv4 Addressing & Subnetting

⭐ CCST GOLD LINE (Remember This First)

Subnetting = Dividing one big network into smaller networks

👉 If you understand this idea, everything below becomes simple ✅

🔢 What is IPv4 Addressing?

An IPv4 address is a 32-bit number used to identify a device on a network.

📌 Written in dotted decimal format

🧠 Example: 192.168.1.10

• 4 parts (octets)
• Each part: 0–255
• Total = 32 bits (8 + 8 + 8 + 8)

🧩 What is a Subnet? (Very Important)

A subnet (sub-network) is a smaller network created from a bigger network.

🧠 Think like this:

🏘️ One big society
Divide it into blocks (A, B, C)
👉 Each block = Subnet
🔹 Why Subnetting is Needed
  • Reduce network traffic
  • Improve performance
  • Better security
  • Easy management
📌 Without subnetting → network becomes noisy & slow

🧱 Subnet Mask (Core Concept)

A subnet mask tells which part of the IP is network and which part is host.

🧠 Example:

IP Address: 192.168.1.10
Subnet Mask: 255.255.255.0

👉 First 3 octets = Network
👉 Last octet = Hosts

Subnet Mask Operation

Subnet Mask Operation
Common Subnet Masks
Subnet MaskMeaning
255.0.0.0Class A
255.255.0.0Class B
255.255.255.0Class C
📌 Most home networks use 255.255.255.0

✂️ CIDR / Slash Notation (VERY IMPORTANT)

CIDR (Classless Inter-Domain Routing) uses slash notation to show how many bits belong to the network.

🧠 Example:

192.168.1.0/24

👉 24 bits = Network
👉 8 bits = Host

📌 /24 = 255.255.255.0

Subnetting & CIDR Visualization

CIDR and Subnetting
Common CIDR Values
CIDRSubnet MaskTotal IPs
/24255.255.255.0256
/25255.255.255.128128
/26255.255.255.19264
/27255.255.255.22432
/28255.255.255.24016
📌 Usable hosts = Total − 2

📣 Broadcast Address (Very Important)

A broadcast address is the last IP in a subnet, used to send data to all devices.

🧠 Example:

Network: 192.168.1.0/24
Broadcast: 192.168.1.255

👉 Message sent to .255 reaches everyone 📢

📡 What is a Broadcast Domain?

A broadcast domain is a group of devices that receive broadcast messages.
📌 Key rule:
• Switch → same broadcast domain
• Router → breaks broadcast domain

Broadcast Domain Example

Broadcast Domain

🧩 Subnetting Example (Step-by-Step – EASY)

🎯 Requirement

Network: 192.168.1.0/24
Need: 4 subnets
🔹 Step 1: Borrow bits
/24 → /26
Subnet Mask: 255.255.255.192

Subnet Division Direction

Subnetting Direction
🔹 Step 2: Subnets Created
Subnet Network Broadcast Usable Hosts
1 192.168.1.0 192.168.1.63 .1 – .62
2 192.168.1.64 192.168.1.127 .65 – .126
3 192.168.1.128 192.168.1.191 .129 – .190
4 192.168.1.192 192.168.1.255 .193 – .254
📌 Each subnet = separate broadcast domain

🛠️ CCST Troubleshooting Scenarios

ProblemLikely Issue
Devices can’t communicateWrong subnet mask
Broadcast traffic too highLarge broadcast domain
IP conflictOverlapping subnets
Internet works, LAN failsSubnet misconfiguration
🧠 Memory Tricks (Exam Gold 🥇)

Subnet = Smaller network
/24 = 255.255.255.0
Broadcast = Last IP
Usable hosts = Total − 2
Router breaks broadcast domain