When you are tasked with designing a network infrastructure, one of the first decisions you'll make is how to divide your IP address space. For decades, the standard method taught in networking fundamentals and CCNA courses has been FLSM (Fixed-Length Subnet Masking).
In this guide, we will break down exactly what FLSM is, step-by-step instructions on how to calculate it, and why it might (or might not) be the right choice for your modern enterprise network.
What is Fixed-Length Subnet Masking (FLSM)?
As the name suggests, FLSM is a subnetting technique where all the subnets created from a major network block are assigned the exact same subnet mask. Because the subnet mask is identical across the board, every single subnet will have the exact same number of available host IP addresses.
Imagine you have a large pizza. FLSM is the equivalent of slicing that pizza into exactly 8 equal slices. It doesn't matter if one person is starving and another person is only slightly hungry; everyone gets the exact same size slice.
The FLSM Formulas You Must Know
To calculate FLSM mentally or on paper, you need to memorize two fundamental binary formulas:
- Calculating Subnets:
2^n(where n is the number of network bits you borrow from the host portion). - Calculating Hosts:
2^h - 2(where h is the number of host bits remaining). We subtract 2 because the first IP is reserved for the Network Address and the last IP is reserved for the Broadcast Address.
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Open the Free FLSM Calculator →Step-by-Step FLSM Calculation Example
Let’s look at a real-world scenario. Your ISP has assigned your company the Class C network block: 192.168.1.0/24. Your IT Director asks you to divide this network into 4 equal subnets for four different departments: HR, Sales, IT, and Guest Wi-Fi.
Step 1: Determine How Many Bits to Borrow
You need 4 subnets. Using our subnet formula (2^n), we need to find an n that gives us at least 4.
If we borrow 1 bit: 2^1 = 2 subnets (Not enough).
If we borrow 2 bits: 2^2 = 4 subnets (Perfect!).
Step 2: Calculate the New Subnet Mask
Your original prefix was /24. Since you borrowed 2 bits to create the new subnets, you simply add them together: 24 + 2 = /26.
A /26 prefix equals a decimal subnet mask of 255.255.255.192.
Step 3: Calculate Hosts Per Subnet
An IPv4 address has 32 bits total. Since our new mask is /26, we have 6 bits remaining for hosts (32 - 26 = 6).
Using the host formula: 2^6 - 2 = 64 - 2 = 62 usable hosts per subnet.
Step 4: Define the Network Boundaries
Because our block size is 64 (from 2^6), our networks will increment by 64. Here is the final FLSM routing table you would configure on your Cisco switches:
| Subnet Name | Network Address | First Usable IP | Last Usable IP | Broadcast Address |
|---|---|---|---|---|
| HR Dept | 192.168.1.0/26 | 192.168.1.1 | 192.168.1.62 | 192.168.1.63 |
| Sales Dept | 192.168.1.64/26 | 192.168.1.65 | 192.168.1.126 | 192.168.1.127 |
| IT Dept | 192.168.1.128/26 | 192.168.1.129 | 192.168.1.190 | 192.168.1.191 |
| Guest Wi-Fi | 192.168.1.192/26 | 192.168.1.193 | 192.168.1.254 | 192.168.1.255 |
The Disadvantage of FLSM: Wasted IPs
While FLSM is incredibly easy to calculate and implement, it has a massive flaw: IP Address Waste.
Look back at our example. Every subnet supports 62 hosts. But what if the IT Department only has 4 employees? You are assigning a block of 62 IP addresses to a room that only needs 4. You have just wasted 58 IP addresses that can never be used by anyone else in the building!
To solve this problem of wasted IP addresses, modern network engineers use Variable-Length Subnet Masking (VLSM), which allows you to create "subnets of subnets" tailored exactly to the size of the department.
If you are working on a complex network design, don't do it by hand. Head over to our All-In-One Subnet Calculator to automatically generate optimized FLSM and VLSM tables in seconds.