VLSM Calculator

The Power of VLSM: Optimizing Your IP Address Space

Published on September 16, 2025

In traditional subnetting, also known as classful subnetting, all subnets in a network must be the same size. This often leads to significant waste of IP addresses. For example, if you have one department needing 100 addresses and another needing only 10, both would get a subnet large enough for 100 hosts, wasting dozens of IPs in the second subnet.

Variable Length Subnet Masking (VLSM) solves this problem. It allows network administrators to use different subnet masks for each subnet, effectively creating subnets of different sizes tailored to specific needs. This is the cornerstone of modern, efficient IP address management.

How VLSM Works: A Real-World Example

Imagine you're a network administrator for a small company and you have the network block 192.168.10.0/24 to work with. You have the following requirements:

  • Engineering: Needs 50 hosts.
  • Sales: Needs 25 hosts.
  • Management: Needs 5 hosts.
  • WAN Link 1: Needs 2 hosts (for point-to-point).
  • WAN Link 2: Needs 2 hosts (for point-to-point).

Step 1: Sort by Size

The most important rule in VLSM is to always start with the largest subnet requirement and work your way down to the smallest. This prevents a situation where you can't fit a large subnet because the address space has been fragmented by smaller ones.

Our sorted list is: Engineering (50), Sales (25), Management (5), WAN 1 (2), WAN 2 (2).

Step 2: Calculate Each Subnet

Using our VLSM Calculator makes this process automatic, but here's the manual logic:

  1. Engineering (50 hosts): We need 50 addresses, plus one for the network ID and one for broadcast. So, 52 total. The smallest power of 2 that can fit 52 is 64 (26). This means we need 6 bits for hosts, leaving 26 bits for the network (32 - 6 = 26).
    • Network: 192.168.10.0/26
    • Mask: 255.255.255.192
  2. Sales (25 hosts): We need 27 addresses. The smallest power of 2 is 32 (25). This needs 5 host bits, so the mask is /27. We start this subnet right after the Engineering one ends (at 192.168.10.63).
    • Network: 192.168.10.64/27
    • Mask: 255.255.255.224
  3. Management (5 hosts): Needs 7 addresses. Smallest power of 2 is 8 (23). Needs 3 host bits, so mask is /29. Starts after Sales ends (at 192.168.10.95).
    • Network: 192.168.10.96/29
    • Mask: 255.255.255.248
  4. WAN Links (2 hosts each): Need 4 addresses each. Smallest power of 2 is 4 (22). Needs 2 host bits, so mask is /30.
    • WAN 1: 192.168.10.104/30
    • WAN 2: 192.168.10.108/30

By using VLSM, we've efficiently allocated addresses for every department without significant waste. For a detailed breakdown of CIDR notation, see our guide on CIDR Explained.