Networking - Subnetting

What is Subnetting?

Subnetting is the process of dividing a large network into smaller, more manageable sub-networks (subnets).
It’s used to efficiently use IP addresses and improve network performance and security.

Why do we need Subnetting?

  1. Efficient IP Address Usage – Prevents waste of IP addresses.

  2. Improved Performance – Smaller networks reduce congestion (less broadcast traffic).

  3. Better Security & Control – Each subnet can be isolated or controlled separately.

  4. Simplifies Management – Easier to organize devices by department, building, or function.

How Subnetting Works

  • An IP address has two parts:

    • Network portion (identifies the network)

    • Host portion (identifies devices in that network)

  • Subnetting borrows bits from the host portion to create more networks.

Example:

Suppose you have:

  • Network: 192.168.1.0

  • Default subnet mask: 255.255.255.0 (a /24 network, 256 addresses, 254 usable hosts)

If you subnet it into /26 networks (255.255.255.192):

  • Each subnet has 64 addresses (62 usable hosts)

  • Subnets created:

    • 192.168.1.0 – 192.168.1.63

    • 192.168.1.64 – 192.168.1.127

    • 192.168.1.128 – 192.168.1.191

    • 192.168.1.192 – 192.168.1.255

So instead of one big 254-host network, you get four smaller networks.

Key Terms in Subnetting

  • Subnet Mask: Defines which part of an IP address is network vs. host.

  • CIDR Notation: Short form (e.g., /24 means 255.255.255.0).

  • Network Address: First address in a subnet (used to identify the subnet).

  • Broadcast Address: Last address in a subnet (used to reach all hosts in that subnet).

  • Usable Hosts: IPs between network and broadcast (assignable to devices).