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?

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?

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?

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?

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?

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)?

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?

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?

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

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

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?

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

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