IP Subnet Calculator
Date Published
Subnetting is one of those things that feels like it should just click but somehow still requires a scratch pad every time. Whether you are allocating address space for a new VLAN, figuring out if two ranges overlap, or trying to remember whether that wildcard mask goes on the ACL the right way round, a quick calculator saves a lot of table lookups. So I built one that covers the full workflow instead of just spitting out a network address and calling it done.
The Calculate tab takes an address in whatever form you have it — /24, 255.255.255.0, or the wildcard form 0.0.0.255 — and gives back the full set: network, broadcast, first and last usable, subnet mask, wildcard, host count, class, address type, integer and hex representations, reverse DNS PTR record, and the reverse zone name. There is also a binary view with the network and host bits colour coded, and navigation buttons to jump to adjacent subnets, the supernet, or either half.
VLSM and route planning
The VLSM planner tab is the one I reach for most when starting a new network layout. You give it a parent block and a list of segments with their host counts, optionally add a growth percentage and a toggle to use /31 for point-to-point links, and it works out the smallest prefix for each segment, sorts them largest first to keep everything aligned, and shows what is left over. The result table has the gateway address, last usable, and a fill percentage so you can see at a glance which segments have room to grow. Everything exports as CSV.
Aggregation, tools, and IPv6
The Summarize tab accepts a mix of CIDRs, single addresses, and hyphenated ranges, merges overlapping entries, and produces a minimal list of aggregate routes plus a single covering supernet. The Tools tab covers the utility jobs: converting an address range to the minimum set of CIDRs, checking whether an address sits inside a subnet, detecting overlaps between two blocks, finding the right prefix for a given host count, and converting between prefix length, dotted mask, and wildcard. The IPv6 tab handles prefix breakdown, splitting into smaller blocks, EUI-64 interface ID generation from a MAC address, and SLAAC address calculation. The Reference tab has the full /0 to /32 prefix table with one-click apply, plus the special-purpose IPv4 and IPv6 ranges. Everything runs in your browser, click any tile to copy the value, and nothing leaves your machine.