How to Subnet Using VLSM

Last Updated: August 17, 2019

One method for subnetting is done using the method known as VLSM or Variable Length Subnet Mask. This is an alternative to CLSM (constant length subnet mask), in that using VLSM you subnet a network by applying a different subnet mask for the various subnet that you create. This method is preferred when the various subnets have substantial differences in the number of hosts that they need per subnet. In situations some subnets may only need a few addresses while others require a lot more.

Using VLSM can accomplish this by wasting as few address as possible.

Suppose you are given the address block of 192.168.10.0/24 and you need the following requirements:
- subnet A ~ 66 hosts
- subnet B ~ 10 hosts
- subnet C ~ 22 hosts
- subnet D ~ 2 hosts

Steps

1
List all of your possible subnets.
- /24 = 254 hosts
- /25 = 126 hosts
- /26 = 64 hosts
- /27 = 32 hosts
- /28 = 16 hosts
- /29 = 6 hosts
- /30 = 2 hosts
2
Arrange your requirements in descending order.
- Subnet A - 66
- Subnet C - 22
- Subnet B - 10
- Subnet D - 2
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3
Assign the appropriate subnet masks to each subnet.
- Allocate the First highest subnet
  - Assign 192.168.10.20 mask /25 for subnet A to satisfy there need for 66 hosts. The last octet of the subnet mask will be 1000000 (255.255.255.128)
- Assign the next highest subnet
  - Assign 192.168.10.128 the mask /27 for subnet C; the last octet of the subnet mask will be 11100000 (255.255.255.224)
- Assign the next highest subnet
  - Assign 192.168.10.160 the mask /28 for subnet B; the last octet of the subnet mask will be 11110000 (255.255.255.240)
- Assign the last subnet
  - Assign 192.168.10.176 the mask /30 for subnet D; the last octet of the subnet mask will be 11111100 (255.255.255.252).
4
Review the Subnetting summary.
- Subnet A = 192.168.10.0/25 ~126 hosts (needed 66)
- Subnet C = 192.168.10.128/27 ~ 30 hosts (needed 22)
- Subnet B = 192.168.10.160/28 ~ 14 hosts (needed 10)
- Subnet D = 192.168.10.176/30 ~ 2 hosts (needed 2)
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Warnings

Be careful when subnetting and make sure that your Address ranges for each subnet are correct

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Make sure that your addressing scheme meets the requirements

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When listing the possible subnets make sure that the number of hosts is the number of USABLE hosts. Use the formula (2^n)-2 where n = number of host bits for that subnet mask. We always subtract 2 to account for the network address and broadcast address!!!

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