VLSM Calculator
Divide a network into subnets of different sizes using variable length subnet masking, allocated largest first so the address space is used efficiently and no ranges overlap.
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Fixed length subnetting wastes most of the space
Splitting a /24 into eight equal /27 subnets gives every segment thirty usable addresses whether it holds two devices or twenty five. A point to point link between two routers needs exactly two addresses and a /27 gives it thirty, so twenty eight are wasted on every such link. Variable length subnet masking allocates each segment only the size it needs, which on a typical network recovers more address space than any other single change.
Allocate largest first or you fragment the space
Subnets must be aligned to their own size, so a /26 can only start at an address divisible by 64. Allocating a small subnet first can leave the remaining space unable to hold a larger one even though the total free addresses are sufficient. Working from largest to smallest avoids this entirely, and it is the reason every VLSM procedure starts by sorting the requirements in descending order.
Two addresses per subnet are never usable
The first address in a subnet identifies the network and the last is the broadcast address, so a /29 with eight addresses provides six for hosts. This catches people at the small end: a /30 gives two usable addresses, which is exactly right for a point to point link and useless for anything else, and a /31 gives none at all under classic rules although RFC 3021 permits it specifically for point to point links.
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Frequently Asked Questions
Why must subnets be allocated largest first?
Because a subnet must start at an address that is a multiple of its own size. Allocating small subnets first leaves the free space fragmented, so a larger subnet may not fit even though enough total addresses remain.
Why does a /29 give six usable addresses rather than eight?
The first address identifies the network and the last is the broadcast address. That reserves two in every subnet, which is why the usable count is always the size minus two.
What size should a point to point link be?
A /30 gives exactly two usable addresses, which is what a link between two routers needs. RFC 3021 also permits a /31 for point to point links, providing two addresses with no network or broadcast reservation, and most modern equipment supports it.
Can I use VLSM with any routing protocol?
It requires a classless protocol that carries the subnet mask in its updates: OSPF, EIGRP, IS-IS and BGP all do. The original RIP version 1 and IGRP do not, which is why VLSM was impossible on those networks.
How much headroom should I leave per subnet?
Enough that growth does not force a renumber, since changing a subnet mask later means reconfiguring every device on it. Doubling the current host count is a common rule, and the calculator shows the spare capacity each allocation leaves.
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How to Use
Enter the parent network and the host count each subnet needs to get an allocation plan.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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