What are the different methods of subnet masking used in networking, and how do they work?

Key takeaways

• Write a mask as default class mask, custom dotted decimal or CIDR, and /24 is the same as 255.255.255.0.
• A mask in binary is a run of ones followed by zeros, so only certain values such as 128, 192, 224 and 240 are valid.
• Use VLSM to give each subnet only the size it needs.

Network administrators and IT professionals need to manage IP addresses efficiently. Subnet masking divides networks into manageable and secure sub-networks. This guide breaks down the methods of subnet masking, using simple explanations, practical examples, and a reference table. It is especially useful if you're preparing for certifications like CCNA or CompTIA Network+.

What Is Subnet Masking?

Subnet masking is a technique used to divide an IP network into smaller segments called subnets. Each subnet functions as a separate network, helping manage traffic, improve security, and reduce congestion.

A subnet mask works with an IP address to determine which part refers to the network and which part refers to the host (individual device). For example:

IP Address: 192.168.1.10
Subnet Mask: 255.255.255.0

This tells the system that the first three octets (192.168.1) identify the network and the last octet (.10) identifies the device.

Why Is Subnet Masking Important?

• Reduces broadcast traffic

• Improves network performance

• Allows IP address reuse across different subnets

• Enhances security by isolating networks

• Optimizes network management

Types of IP Address Classes & Default Subnet Masks

Common Methods of Subnet Masking

Let’s explore the most widely used subnetting techniques:

1. Fixed Length Subnet Masking (FLSM)

Definition:
FLSM divides an IP address space into equal-sized subnets using the same subnet mask for all.

Example:
If you need 4 equal subnets from a Class C network (e.g., 192.168.1.0/24), you can apply:

Subnet mask: 255.255.255.192 (/26)
Subnets: 4 (each with 64 IPs)

Use Case:
Ideal when you need consistent subnet sizes, like dividing a network across equal-sized departments.

2. Variable Length Subnet Masking (VLSM)

Definition:
VLSM allows subnets of different sizes by applying different masks. It utilizes IP addresses more efficiently.

Example:
One department needs 50 IPs, another only needs 10. Instead of wasting IPs, VLSM gives each subnet just what it needs.

Subnet A: 255.255.255.192 (/26) for 64 IPs
Subnet B: 255.255.255.240 (/28) for 16 IPs

Use Case:
Perfect for large enterprises or ISPs with diverse network requirements.

3. CIDR (Classless Inter-Domain Routing)

Definition:
CIDR discards the traditional class-based system (A/B/C) and uses a slash notation to define subnets (e.g., /24, /27).

Example:

192.168.10.0/22 → Combines four Class C networks into one large subnet

Use Case:
CIDR is used by ISPs and cloud providers to aggregate IPs and reduce the size of routing tables.

Understanding CIDR Notation and Bit Masking

Real-World Example: Subnetting a Class C Network

Scenario:
You’re assigned a network 192.168.1.0/24 and need to divide it into 4 departments with up to 50 users each.

Step-by-Step:

1. Required hosts per subnet = 50

2. Nearest power of 2 = 64 → Needs 6 bits for hosts

3. Subnet mask: /26 → 255.255.255.192

Subnets created:

• 192.168.1.0/26 → Hosts 1–62

• 192.168.1.64/26 → Hosts 65–126

• 192.168.1.128/26 → Hosts 129–190

• 192.168.1.192/26 → Hosts 193–254

Tools for Subnet Masking

Here are some handy tools to calculate and validate subnetting:

Key Takeaways

• Subnet masking helps divide networks efficiently for performance, management, and security.

• Methods like FLSM, VLSM, and CIDR offer flexibility in IP distribution.

• Knowing CIDR notation and bitwise operations helps in advanced network design.

• Tools like IP calculators simplify the planning process.

Conclusion

Understanding the methods of subnet masking is foundational for anyone working in IT infrastructure, network engineering, or cybersecurity. Whether you’re managing cloud networks or planning office LANs, these techniques help allocate IPs efficiently, secure networks, and support scaling.

To take this further with guided labs and an instructor, see our CCNA 200-301 exam preparation.

Related reading

• [2026] Top 10 CCNA Interview Questions
• [2026] CCNA Interview Questions with Detailed Answers
• What Is Basic Networking?

Reference

For the authoritative details, see IETF RFCs.