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

In today's digital world, managing IP addresses efficiently is a crucial skill for network administrators and IT professionals. Subnet masking plays a central role in dividing 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 to help you master the concept—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.

 FAQs 

What is subnet masking in networking?

Subnet masking is a process used to divide an IP network into smaller, manageable sub-networks or subnets.

Why do we use subnet masks?

We use subnet masks to identify which portion of an IP address is the network and which is the host, allowing better routing and address management.

What are the main types of subnet masking?

The main types are Fixed Length Subnet Masking (FLSM), Variable Length Subnet Masking (VLSM), and CIDR (Classless Inter-Domain Routing).

What is FLSM?

FLSM (Fixed Length Subnet Masking) divides a network into equal-sized subnets using the same subnet mask.

What is VLSM?

VLSM (Variable Length Subnet Masking) allows subnets of different sizes, optimizing IP address usage.

What is CIDR?

CIDR stands for Classless Inter-Domain Routing and uses slash notation (e.g., /24) to define custom subnet masks.

What is the default subnet mask for Class C?

255.255.255.0 is the default subnet mask for Class C IP addresses.

How does a subnet mask work?

It uses a 32-bit binary number to mask part of an IP address, determining the network and host portions.

What is the difference between FLSM and VLSM?

FLSM uses equal-sized subnets; VLSM uses variable-sized subnets to reduce waste.

What does /24 mean in CIDR notation?

/24 means 24 bits are used for the network, and the subnet mask is 255.255.255.0.

How many hosts can a /26 subnet support?

A /26 subnet supports 62 usable hosts.

What is the smallest usable subnet?

The smallest usable subnet is /30, which supports 2 hosts.

What is subnetting used for in real networks?

Subnetting improves network performance, security, and IP address management.

Can I mix FLSM and VLSM in the same network?

Yes, but it’s more complex and usually requires careful routing and planning.

Is CIDR better than traditional classful addressing?

Yes, CIDR is more efficient and flexible than class-based addressing.

What are private IP ranges for subnetting?

Common ranges: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

What is a subnet mask of 255.255.255.240?

It’s a /28 subnet mask, supporting 14 usable hosts.

How to calculate the number of hosts in a subnet?

Use the formula: 2^(32 - subnet bits) - 2.

What is supernetting?

Supernetting combines multiple networks into a larger one—opposite of subnetting.

What tools help with subnet masking?

Tools like IPCalc, SubnetOnline, and SolarWinds IPAM are commonly used.

What happens if subnetting is done incorrectly?

It may cause IP conflicts, unreachable networks, or routing issues.

What is broadcast address in subnetting?

It is the last IP address in a subnet and used for broadcasting to all hosts.

Can subnetting be applied in IPv6?

Yes, but IPv6 uses prefix lengths and works differently than IPv4.

What is a /29 subnet used for?

A /29 subnet allows 6 usable hosts—commonly used for point-to-point links.

How do you find the subnet mask from CIDR?

Convert the number of bits (e.g., /26) into a dotted decimal format.

What is a wildcard mask?

Used in ACLs, a wildcard mask is the inverse of a subnet mask.

What is the purpose of network ID?

The network ID identifies the subnet and is used by routers to route packets.

How do routers use subnet masks?

Routers use them to determine the best route for a packet.

Is subnetting required in small networks?

Not always, but it can still help in logically organizing devices.

Can you use subnetting in wireless networks?

Yes, especially in enterprise environments to separate SSIDs or VLANs