// what it does and why
An IPv4 subnet calculator with one input field. Type an address and a prefix, get the network, the broadcast, the mask and the usable range. It recalculates as you type and it never contacts a server.
Open it →Type something like 192.168.1.10/24. There is no calculate button, because it runs on every keystroke and the answer is already there by the time you stop typing. Leave the prefix off entirely and it assumes a /24.
Ten values come back: network, broadcast, subnet mask, wildcard mask, first and last usable host, usable host count, total addresses, the prefix, and whether the block is private space or public. The wildcard is in there because access lists on network gear want the mask inverted, and inverting it in your head at the wrong moment is a good way to open something you did not mean to open.
Under that is a binary panel with the address, the mask and the network lined up, and a line telling you how many bits belong to the network and how many are left for hosts. Four preset tiles sit above the field: the three private ranges, plus a /26, which is the smallest split most people actually meet.
I work in IT, and what comes up is never hard arithmetic. It is a five second answer while somebody waits on the phone. What is the broadcast on this /26. How many hosts do I actually get. Is that address private space, or did someone put a public one inside the network.
The calculators that already exist all work. They are also buried in ads, or they make you assemble the question out of six dropdowns, or they post your address to a server to do maths a browser finishes instantly. I wanted one page that is ready before I finish tabbing to it.
There is no backend. Every value is arithmetic on a single 32 bit number, done inside the page, so the calculation itself makes no requests and the address you type stays on your machine.
That matters more than it sounds. Internal addressing is exactly the sort of thing people paste into a random website without a second thought, and a list of your internal ranges is a small free map of your network. Nobody should have to trust me with it.
A /32 is a single host and a /31 is a two address point to point link. Neither one reserves a network address and a broadcast address the way everything from a /30 down does, and the /31 case was standardised specifically so that router to router links stop wasting half of a four address block.
So this tool reports no broadcast for those two and counts every address as usable, rather than announcing zero usable hosts on a link that is carrying traffic right now. It says so under the panel, because whoever lands on that case is usually the person who most needs to know why.
Most calculators hand you a result and stop. This one shows the bits underneath it, because the answer stops being something to memorise once you can see where the mask ends and the host part begins.
IPv4 only. No IPv6, no splitting a block into a list of smaller subnets, and no planning table for carving one range into several differently sized ones. Those are all reasonable things to want and none of them are here yet. It answers one question properly and stops.