How to Use This Tool
Enter a block and the subnets you need. They are allocated largest first, which is what makes VLSM work, and the table shows exactly where each one lands.
The alignment rule
A subnet has to start on a multiple of its own size. A /25 holds 128 addresses and can only begin at .0 or .128. A /26 holds 64 and can begin at .0, .64, .128 or .192.
That constraint is why order matters. Place a small subnet at the start of a block and it may occupy the only boundary a larger subnet could have used. There is then plenty of free space and nowhere legal to put the large block — the failure is alignment, not capacity, which is why it confuses people the first time they meet it.
Allocating largest first avoids the problem entirely, because a large block placed at a valid boundary leaves valid boundaries for everything smaller. It is a one-line rule with a real proof behind it.
Why every subnet loses two addresses
The first address in a subnet is the network address and the last is the broadcast address, so a /24 gives 254 usable hosts rather than 256. That is why the planner asks how many hosts you need and then finds a prefix with room for them plus two.
A link between two routers therefore needs a /30 — four addresses, two usable — even though only two hosts exist. Unless you enable the /31 option: RFC 3021 allows a two-address subnet with no network or broadcast on point-to-point links, saving two addresses per link. Across a large network that adds up, and most modern routers support it.
How much headroom to add
Because sizes are powers of two, a small increase in requirement sometimes costs nothing and sometimes doubles the allocation. Going from 60 hosts to 62 stays in a /26; going from 62 to 63 needs a /25 and doubles the block.
So look at the waste column before padding your numbers. If you are already near the bottom of a tier, extra headroom is free. If you are near the top, it is expensive, and it is worth asking whether the requirement is real.
Practical notes
- Leave room at the end. Allocating a block exactly to capacity means the next requirement needs renumbering. Reserving a quarter is cheap insurance in private address space, where addresses cost nothing.
- Group by function, not by size. Adjacent subnets that belong to the same purpose can be summarised into one route later; scattered ones cannot.
- Document as you go. The table here is exportable, and a subnet plan nobody wrote down is rediscovered by whoever breaks it.
