SubnetCalculator.org
CCNA & Enterprise Network Design Tool

VLSM Subnet Calculator

Design optimal subnets tailored to your exact host counts. Auto-allocates non-overlapping CIDR blocks, calculates wasted host space, and exports results.

Starting address of your major parent network block

Overall parent network capacity

Subnet Requirements (Departments / Segments)

Enter department names and the minimum number of usable hosts required.

Swipe requirements horizontally
#Subnet / Department NameNeeded HostsAction
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2
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Auto-sorts by size descending
Needed Hosts
92
Allocated Hosts
108
Wasted Hosts
16
Remaining Hosts
146

Allocated VLSM Subnets

Optimally assigned subnets in ascending network address order.

Swipe table horizontally
DepartmentNeededAllocatedSubnet (CIDR)NetmaskUsable Host RangeBroadcast IP

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Frequently Asked Questions

Clear answers to common questions about subnetting, CIDR notation, and network design.

What is VLSM (Variable Length Subnet Masking)?
VLSM (Variable Length Subnet Masking) is an advanced IP allocation method where subnets within the same network can have different subnet masks. Instead of dividing a network into equally sized blocks (FLSM - Fixed Length Subnet Masking), VLSM customizes each subnet size to match the exact host requirements of each department or link, preventing waste of precious IP space.
How does the VLSM allocation algorithm work?
The fundamental rule of VLSM is to sort host requirements from largest to smallest. Subnets requiring the most hosts must be allocated first to align with power-of-2 address boundaries. Subsequent smaller subnets are packed neatly into the remaining address space without creating routing overlaps or fragmentation.
Why is VLSM essential for CCNA and network design?
In real-world networks, different segments require vastly different capacities: a LAN floor may require 100 hosts, while a point-to-point router link requires only 2 hosts. Using a uniform /24 on both would waste 252 addresses on the router link. VLSM allocates a /25 for the LAN and a /30 for the link, achieving near 100% address utilization.
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