What is the difference between VLSM and FLSM?

For subnetting an IP, we use two approaches:

• Fixed length subnet mask (FLSM)
• Variable length subnet mask (VLSM)

FLSM and VLSM refer to how IP address space is allocated within each organization.

Note: To read about IP addressing and subnetting, click here.

FLSM

Fixed-length subnet mask (FLSM) is a subnet deployment strategy in which a block of IP addresses is split into several subnets of identical length. It is also known as classful subnetting.

Network design

The following topology divides the network into two identical subnets comprising the following:

• $60$ hosts
• $110$ hosts

Although fixed-length subnetting assists in efficiently utilizing the IP addresses, this architecture can be improved using VLSM.

VLSM

Variable-length subnet mask (VLSM) refers to a strategy that allows all the subnetworks to have variable sizes. Network administrators can use VLSM subnetting to divide an IP address space into subnets of various lengths and assign them based on the needs of the network. It is also called classless subnetting.

Network design

The following network design is divided into two subnets but of variable sizes. The sole purpose is to waste fewer IPs, thus improving network speed and performance.

The differences between FLSM and VLSM are as follows: