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📘 08-03: Subnet Mask

Network Systems

Module 08: Subnets and VLANs

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❓ What is a Subnet Mask?

A subnet mask is used to divide an IP address into two parts:

  • Network portion
  • Host portion

It defines where the network ends and the host begins.

Note: A common mistake for beginners is to confuse the binary form of a subnet mask with an IP address.
A subnet mask is NOT an IP address.
Its only function is to determine which portion of an IP address represents the network and which portion represents the host.

🧮 How Subnet Mask is Represented

There are three ways to represent a subnet mask:

🔹 1. Binary Representation

Example: Subnet Mask: 11111111.11111111.00000000.00000000

A subnet mask uses binary values:

  • 1s → Network portion
  • 0s → Host portion

In this example: - First 16 bits → Network
- Last 16 bits → Host

🔹 2. Decimal Representation

Example: Subnet Mask: 255.255.0.0

The decimal form is a human-readable version of the binary mask.

To understand network and host portions, it must be interpreted in binary.

🔹 3. CIDR Notation

Subnet masks are often written using CIDR notation.

Examples: - /24 → First 24 bits are the network portion
- /16 → First 16 bits are the network portion
- /8 → First 8 bits are the network portion

💡 Why Subnet Mask is Important

  • Defines network boundaries
  • Enables communication within networks
  • Supports network segmentation
  • Helps in routing decisions

💡 Key Idea

An IP address alone is not enough.

👉 A subnet mask tells: - which part is the network
- which part is the host