VLSM Calculator
Divide a network into variable-length subnets, or summarize networks into a single route.
Segments needed
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Frequently asked questions
- What is VLSM?
- Variable Length Subnet Masking lets you divide a network into subnets of different sizes based on how many hosts each segment actually needs, instead of splitting it into equal fixed-size blocks and wasting addresses.
- Why does the order of allocation matter?
- VLSM allocates the largest requirements first. Allocating smaller blocks before larger ones can fragment the address space so that a later, bigger request no longer fits, even if enough total addresses remain.
- What is route summarization?
- Route summarization (or supernetting) is the reverse of VLSM: it combines several smaller networks into a single, larger network announcement, reducing the number of routes a router needs to track.
- Why is my summary marked as approximate?
- A summary is approximate when the smallest block that contains all the given networks also includes addresses that weren't part of any of them. This is common when the input networks don't align on a power-of-two boundary.
- Can VLSM subnets overlap with each other?
- No. This calculator allocates each subnet from the next available address after the previous one, so allocations never overlap as long as they all fit inside the base network you provide.
- What happens if my segments don't fit in the base network?
- The calculator will tell you the segments don't fit and suggest using a larger base network or reducing the number of hosts requested per segment.