VLSM Subnetting Examples and Calculation Explained

There are two types of subnetting: FLSM and VLSM. FSLM subnetting uses the same subnet mask for all subnets. VLSM subnetting allows us to choose subnet masks based on the network requirement.

VLSM extends FLSM. We first use FLSM to break the given subnet into the same-sized subnets. Then, we use FLSM again on created subnets to break them into smaller same-sized subnets. We repeat this process until we get the subnets that meet our requirements. In FLSM, we use subnetting charts. A subnetting chart tells us the number of subnets we can get from a given IP subnet having a specific number of host addresses.

Class A Subnetting chart

Class B Subnetting chart

Class C Subnetting chart

VLSM Example

The following image shows an example network. We will perform VLSM subnetting for this network.

VLSM Subnetting

In the first step, we arrange all segments in descending order based on their requirements.

Each segment needs two additional IP addresses: one for the network address and another for the broadcast address. Because of this, we must select a block size that provides more or equal IP addresses to the actual requirement for each segment.

Actual requirement =  Total hosts + two (network and broadcast)

In the next step, we find the subnet mask in the slash notation that provides the required block size. Our first and second segments need a block size of 32. The subnet mask /27 provides a total of 8 subnets having 32 IP addresses each.

0-31, 32-63, 64-95, 96-127, 128-159, 160-191, 192-223, 224-255

We can assign the first two subnets to these segments.

0-31, 32-63, 64-95, 96-127, 128-159, 160-191, 192-223, 224-255

The next two segments need a block size of 16. The subnet mask /28 provides 16 subnets containing 16 IP addresses each.

0-15, 16-31, 32-47, 48-63,64-79, 80-95, 96-111, 112-127, 128-143, 144-159, 160-175, 176-191, 192-207, 208-223, 224-239, 240-255

We cannot use the first four subnets. They contain the addresses we have already assigned to the first and second segments. We can use the fifth and sixth subnets for the third and fourth LAN segments.

0-15, 16-31, 32-47, 48-63,64-79, 80-95, 96-111, 112-127, 128-143, 144-159, 160-175, 176-191, 192-207, 208-223, 224-239, 240-255

The next four segments need a block size of 4. The subnet mask /30 provides 64 subnets containing 4 IP addresses.

0-3, 4-7, 8-11, 12-15, 16-19, 20-23, 24-27, 28-31, 32-35, 36-39, 40-43, 44-47, 48-51, 52-55, 56-59, 60-63, 64-67, 68-71, 72-75, 76-79, 80-83, 84-87, 88-91, 92-95, 96-99, 100-103, 104-107, 108-111, 112-115, 116-119, 120-123, 124-127, 128-131, 132-135, 136-139, 140-143, 144-147, 148-151, 152-155, 156-159, 160-163, 164-167, 168-171, 172-175, 176-179, 180-183, 184-187, 188-191, 192-195, 196-199, 200-203, 204-207, 208-211, 212-215, 216-219, 220-223, 224-227, 228-231, 232-235, 236-239, 240-243, 244-247, 248-251, 252-255

The first 24 subnets contain the IP addresses we assigned in the previous segments. We can use the subnets 25, 26, 27, and 28 for these segments.

0-3, 4-7, 8-11, 12-15, 16-19, 20-23, 24-27, 28-31, 32-35, 36-39, 40-43, 44-47, 48-51, 52-55, 56-59, 60-63, 64-67, 68-71, 72-75, 76-79, 80-83, 84-87, 88-91, 92-95, 96-99, 100-103, 104-107, 108-111, 112-115, 116-119, 120-123, 124-127, 128-131, 132-135, 136-139, 140-143, 144-147, 148-151, 152-155, 156-159, 160-163, 164-167, 168-171, 172-175, 176-179, 180-183, 184-187, 188-191, 192-195, 196-199, 200-203, 204-207, 208-211, 212-215, 216-219, 220-223, 224-227, 228-231, 232-235, 236-239, 240-243, 244-247, 248-251, 252-255

The following image shows the assigned IP subnets to each segment.

The following table lists a summary of all subnets.