Subnetting (FLSM & VLSM) – Complete Guide

🔹 Subnetting Basics

✅ What is Subnetting?

Subnetting divides a larger IP network into smaller subnetworks (subnets), improving network performance, security, IP address usage, and simplifying management.

🔸 Purpose and Advantages:

Efficient utilization of IP addresses.
Reduced broadcast traffic.
Easier troubleshooting.
Enhanced security via network isolation.

🔹 IP Address Structure

An IPv4 address consists of two parts:

Network Portion: Identifies the network.
Host Portion: Identifies individual devices.

✅ Classes and Default Masks

Class	Range	Default Mask
A	0.0.0.0 – 127.255.255.255	255.0.0.0
B	128.0.0.0 – 191.255.255.255	255.255.0.0
C	192.0.0.0 – 223.255.255.255	255.255.255.0

🔹 Subnet Mask

A subnet mask defines the network and host portions. Written as dotted decimal (e.g., 255.255.255.0).

🟩 Fixed-Length Subnet Mask (FLSM)

✅ Definition:

FLSM uses the same subnet mask for all subnets, creating equal-sized networks.

✅ FLSM Example Problem:

Subnet 192.168.10.0/24 into 4 equal subnets:

Subnet	Network ID	Usable Hosts	Broadcast Address
1	192.168.10.0/26	.1 – .62	.63
2	192.168.10.64/26	.65 – .126	.127
3	192.168.10.128/26	.129 – .190	.191
4	192.168.10.192/26	.193 – .254	.255

🟦 Variable-Length Subnet Mask (VLSM)

✅ Definition:

VLSM allows subnets of different sizes within the same network, providing efficient IP address allocation.

✅ VLSM Example Problem:

Given 192.168.20.0/24, subnet it as:

Subnet	Hosts Required	Subnet Mask	CIDR	Usable Hosts	Subnet Range
A	100	255.255.255.128	/25	126	192.168.20.0 – .127
B	50	255.255.255.192	/26	62	192.168.20.128 – .191
C	10	255.255.255.240	/28	14	192.168.20.192 – .207

📘 CIDR & Supernetting

CIDR: Allows flexible subnet masks (e.g., /24, /26).
Supernetting: Combines multiple networks into one larger network for route summarization.

🎯 When and How to Use:

Technique	Use-case Scenario
FLSM	Simple networks, equal-sized subnets.
VLSM	Complex networks with different-sized subnets.
CIDR	Internet routing and ISP environments.
Supernetting	Route summarization to reduce routing table size.

✅ Exercises

Exercise 1: FLSM Subnetting

Subnet 192.168.10.0/24 into 4 equal subnets:

Subnet #	Network ID	Usable IP Range	Broadcast Address
1
2
3
4

Exercise 2: VLSM Subnetting

Given 172.16.20.0/24, subnet to meet the following:

Subnet	Required Hosts	Network ID	CIDR	Subnet Mask	Usable IP Range	Broadcast Address
A	50
B	30
C	12
D	6

Subnetting (FLSM & VLSM) Quiz

1. What is the main purpose of subnetting?

A. To increase broadcast traffic B. To divide a large network into smaller logical subnets C. To combine multiple networks into one D. To change IP addresses frequently

Correct answer is B. Subnetting divides a large IP network into smaller subnets for improved performance and management.

2. What is the default subnet mask for a Class B IP address?

A. 255.255.0.0 B. 255.0.0.0 C. 255.255.255.0 D. 255.255.255.255

Correct answer is A. The default mask for Class B addresses is 255.255.0.0.

3. In Fixed-Length Subnet Masking (FLSM), what is true about subnet sizes?

A. Subnets have variable sizes B. Subnets are only for Class A networks C. Subnets change dynamically D. All subnets have the same subnet mask and size

Correct answer is D. FLSM uses the same subnet mask for all subnets, creating equal-sized subnets.

4. What is the increment (block size) for subnetting 192.168.10.0/24 into 4 subnets using FLSM?

A. 32 B. 128 C. 64 D. 16

Correct answer is C. The increment is calculated as 256 minus subnet mask value; here, 256 - 192 = 64.

5. Which subnet mask corresponds to a /25 CIDR notation?

A. 255.255.255.128 B. 255.255.255.192 C. 255.255.255.224 D. 255.255.255.240

Correct answer is A. The /25 prefix length corresponds to subnet mask 255.255.255.128.

6. What is a key advantage of Variable-Length Subnet Masking (VLSM)?

A. Simplifies subnetting calculations B. Efficient use of IP address space by creating variable sized subnets C. Uses the same subnet mask for all subnets D. Eliminates the need for subnet masks

Correct answer is B. VLSM allows subnets of different sizes to optimize IP address usage.

7. Which subnet mask allows for 14 usable hosts per subnet?

A. 255.255.255.128 B. 255.255.255.192 C. 255.255.255.224 D. 255.255.255.240

Correct answer is D. A subnet mask of 255.255.255.240 (/28) provides 14 usable host addresses.

8. What does CIDR notation /26 signify in terms of subnet mask?

A. 255.255.255.128 B. 255.255.255.224 C. 255.255.255.192 D. 255.255.255.240

Correct answer is C. CIDR /26 corresponds to subnet mask 255.255.255.192.

9. Which of the following is an example of supernetting?

A. Combining four /24 networks into one /22 network B. Dividing a network into multiple smaller subnets C. Using variable length subnet masks D. Assigning static IP addresses

Correct answer is A. Supernetting aggregates smaller networks into a larger one, e.g., combining four /24 into a /22.

10. Which subnetting technique is best suited for networks requiring different-sized subnets?

A. Fixed-Length Subnet Mask (FLSM) B. Variable-Length Subnet Mask (VLSM) C. Classful subnetting D. Default subnetting

Correct answer is B. VLSM allows subnets of variable sizes to efficiently allocate IP space according to need.

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