VLSM (Variable-Length Subnet Mask)

A network can be divided into multiple smaller networks. Two of the division methods are called Fixed-Length Subnet Mask (FLSM) and Variable-Length Subnet Mask (VLSM).

VLSM works by dividing the network as small as possible, while FLSM divides the network into equal subnets.

VLSM is more efficient because it only divide subnets into its required sizes.

VLSM is divided into several steps:

(Optional) Find the network address.
Sort the division demands by the Endpoint demand, from largest to smallest.
Divide the network subnet according to each of their Endpoint demands.
Assign each network.

Example

Suppose an organization with the network address 192.168.100.0/24 wants to divide the network into 3 parts:

Finance, with 30 Endpoints.
Marketing, with 15 Endpoints.
IT, with 60 Endpoints.

(Optional) Find the network address

Since we already know the network address is 192.168.100.0/24, we can skip this step.

Sort the division demands by the Endpoint demand, from largest to smallest

We will sort the organization parts by their endpoint demands, like so:

IT, with 60 Endpoints.
Finance, with 30 Endpoints.
Marketing, with 15 Endpoints.

This is done to preserve the division of the network, since VLSM needs to be done decrementally. If it is not done decrementally, it would be very difficult to calculate.

Divide the network according to each of their Endpoint demands

We will start from the largest to the smallest.

IT

The endpoint demand is 60 endpoints. Therefore, we need a subnet that could support 60 endpoints + 1 network address + 1 broadcast address.

We can use the formula E+2 <= 2^n, where E is the amount of endpoints, and n is the smallest wildcard value.

Since E = 60, we can determine that n >= 6.

To determine the subnet, we can simply subtract 32 with the smallest n, where 32-6 = /26.

Finance

The endpoint demand is 30 endpoints. Therefore, we need a subnet that could support 30 endpoints + 1 network address + 1 broadcast address.

We can use the formula E+2 <= 2^n, where E is the amount of endpoints, and n is the smallest wildcard value.

Since E = 30, we can determine that n >= 5.

To determine the subnet, we can simply subtract 32 with the smallest n, where 32-5 = /27.

Marketing

The endpoint demand is 15 endpoints. Therefore, we need a subnet that could support 15 endpoints + 1 network address + 1 broadcast address.

We can use the formula E+2 <= 2^n, where E is the amount of endpoints, and n is the smallest wildcard value.

Since E = 15, we can determine that n >= 5.

To determine the subnet, we can simply subtract 32 with the smallest n, where 32-5 = /27.

Assign each network

Now that we have found the correct subnets, we can assign each network into their respective parts.

IT

We know that the subnet /26 can support 64 Addresses (from 2^n), this means every network could support 64 addresses.

Subnet Mask	`255.255.255.192 (/26)`
Network Address	`192.168.100.0`
Usable IP Range	`192.168.100.1 - 192.168.100.62`
Broadcast Address	`192.168.100.63`

Finance

We know that the subnet /27 can support 32 Addresses (from 2^n), this means every network could support 32 addresses.

Subnet Mask	`255.255.255.224 (/27)`
Network Address	`192.168.100.64`
Usable IP Range	`192.168.100.65 - 192.168.100.94`
Broadcast Address	`192.168.100.95`

Marketing

We know that the subnet /27 can support 32 Addresses (from 2^n), this means every network could support 32 addresses.

Subnet Mask	`255.255.255.224 (/27)`
Network Address	`192.168.100.96`
Usable IP Range	`192.168.100.97 - 192.168.100.126`
Broadcast Address	`192.168.100.127`

Note

Gateway is included within the Usable IP Range.