FLSM (Fixed-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).
FLSM works by dividing the network into equal subnets, while VLSM divides the network as small as possible.
FLSM is easier to calculate because the subnets are equal.
FLSM is divided into several steps:
(Optional) Find the network address.
Divide the network
subnetby the largest Endpoint demand.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.
Divide the network by the largest Endpoint demand
The largest 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.
Assign each network
Now that we have found the correct subnet, we can assign each network into their respective parts.
We know that the subnet /26 can support 64 Addresses (from 2^n), this means every network could support 64 addresses.
Finance
Subnet Mask |
|
Network Address |
|
Usable IP Range |
|
Broadcast Address |
|
Marketing
Subnet Mask |
|
Network Address |
|
Usable IP Range |
|
Broadcast Address |
|
IT
Subnet Mask |
|
Network Address |
|
Usable IP Range |
|
Broadcast Address |
|
Note
Gateway is included within the Usable IP Range.