FLSM Subnetting and VLSM Subnetting
There are two types of Subnetting: FLSM and VLSM. In FLSM, all subnets have an equal number of host addresses and use the same subnet mask. In VLSM, subnets have a flexible number of host addresses. They use a subnet mask based on the number of hosts.
Default IP subnets have a large number of IP addresses. Most networks do not need too many IP addresses. If they use the default subnet, all unused IP addresses become useless. To utilize free IP addresses, subnetting is used.
Subnetting breaks default IP subnets. There are two types of subnetting: FLSM and VLSM.
FLSM stands for Fixed Length Subnet Mask. In it, you break the default IP subnet into subnets of the same size. For example, if the default subnet has 12 IP addresses, you can break it into three equal-sized subnets with 4 IP addresses in each.
VLSM stands for Variable Length Subnet Mask. In it, you break the default IP subnet into subnets of various sizes. For example, if the default subnet has 12 IP addresses, you can split it into two subnets: the first with 8 IP addresses and the second with 4 IP addresses.
Differences between FLSM Subnetting and VLSM Subnetting
The following table lists the differences between FLSM and VLSM.
| FLSM (Fixed Length Subnet Masks) Subnetting | VLSM (Variable Length Subnet Masks) Subnetting |
| All subnets are equal in size. | Subnets are variable in length. |
| All subnets have an equal number of hosts. | Subnets have a variable number of hosts. |
| All subnets use the same subnet mask. | Subnets use different subnet masks. |
| It is easy to configure and manage. | It is complex in configuration and administration. |
| It wastes a lot of IP addresses. | It wastes the minimum IP addresses. |
| It is also known as classful subnetting. | It is also known as classless subnetting. |
| It supports both classful and classless routing protocols. | It supports only classless routing protocols. |
This tutorial is part of the tutorial series "IP Addressing, and Subnetting in Computer Networks Explained with examples". Other parts of this series are the following.
Chapter 01 Introduction to Subnetting
Chapter 02 Network Address Basic Concepts Explained with Examples
Chapter 03 The Subnet Mask and Slash Notation
Chapter 04 Converting Decimal IP Addresses to Binary and Binary to Decimal
Chapter 05 Basic Subnetting in Computer Networks Explained
Chapter 06 Subnetting Tutorial - Subnetting Explained with Examples
Chapter 07 Subnetting Tricks: Subnetting Made Easy with Examples
Chapter 08 FLSM Subnetting and VLSM Subnetting
Chapter 09 VLSM Subnetting Explained with Examples
Chapter 10 VLSM Subnetting Examples and Calculation Explained
Chapter 11 Route Summarization Advantages and Disadvantages
Chapter 12 Supernetting Tutorial: - Supernetting Explained with Examples
Conclusion
This tutorial compared FLSM and VLSM and listed their differences. Learning these differences helps you select the appropriate subnetting method for your network.
By ComputerNetworkingNotes Updated on 2026-05-10