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 you to choose subnet masks based on the network requirement. VLSM extends FLSM. This tutorial explains how VLSM works and how to use it to create IP subnets based on requirements.

VLSM is a repeated FLSM. You first use FLSM to split the given subnet into equally sized subnets. Then, you use FLSM again on the created subnets to break them into smaller, same-sized subnets. You repeat this process until you get the subnets that meet your requirements.

FLSM, use subnetting charts. A subnetting chart tells you the number of subnets you 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 a network. Perform VLSM subnetting for this network.

VLSM Subnetting

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, you must select a block size that provides IP addresses equal to or greater than the actual requirement for each segment.

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

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

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

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, each containing 16 IP addresses.

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

You cannot use the first four subnets. They contain the addresses you have already assigned to the first and second segments. 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 you have assigned to the previous segments. You 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 summarizes all subnets.

Conclusion

VLSM (Variable Length Subnet Mask) is a method of subnetting. It allows you to create subnets of different sizes. This tutorial explained VLSM subnetting using an example.