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📘 08-09: Implementing Subnets on a Network

Network Systems

Module 08: Subnets and VLANs

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💡 Why Implement Subnets?

Subnetting is not only a mathematical process, but also a practical network design technique.

It is used to: - divide a large network into smaller subnetworks
- reduce broadcast traffic
- improve network performance
- enhance security

Each subnet forms a separate broadcast domain.

📌 Basic Network Structure

Subnets 1, 2, and 3 and their respective default gateways

A subnetted network typically consists of:

🔹 Router

  • connects multiple subnets
  • each interface represents a different subnet

🔹 Switch

  • connects devices within a subnet

🔹 Hosts (PCs, printers)

  • assigned IP addresses within a subnet

🧮 Subnet Assignment Example

Example:

  • Subnet 1: 192.168.89.0/27
  • Subnet 2: 192.168.89.32/27
  • Subnet 3: 192.168.89.64/27

👉 Each subnet must have a unique network ID.

📌 Default Gateway

Each subnet requires a default gateway to communicate with other networks.

  • The default gateway is usually the router interface IP

Example: - Subnet 1 → Gateway: 192.168.89.1
- Subnet 2 → Gateway: 192.168.89.33
- Subnet 3 → Gateway: 192.168.89.65

🧮 Communication Between Subnets

  • Devices within the same subnet communicate directly
  • Devices in different subnets communicate through a router

📌 One Router Connecting Multiple LANs

One router connecting multiple LANs

A single router can connect multiple LANs: - Each LAN is assigned a different subnet
- Each subnet is isolated from others
- The router enables communication between subnets

Common use cases: - office networks
- campus networks
- multi-floor buildings

🧮 DHCP Across Subnets

🔹 Problem

DHCP uses broadcast messages, which do not cross routers.

DHCP Relay Agent

📡 Solution: DHCP Relay Agent

A DHCP relay agent allows DHCP communication across subnets.

📡 How DHCP Relay Works

  1. A client sends a DHCP request (broadcast)
  2. The router receives the request
  3. The router forwards it to the DHCP server (unicast)
  4. The DHCP server assigns an IP address

📌 Summary

  • Subnet → logical division of a network
  • Router → connects subnets
  • Switch → connects devices within a subnet
  • Default gateway → exit point of a subnet
  • DHCP relay → enables DHCP across subnets

📌 Key Insight

Subnetting transforms a large network into smaller, manageable networks, enabling efficient communication and better network design in real-world environments.