How to use the subnet calculator
Type an address in CIDR form such as 10.20.30.40/22, or type the address on its own and pick the prefix or the dotted mask from the menus; the two menus mirror each other, so choosing 255.255.240.0 sets /20 and vice versa. The results update as you type. The cards show the network and broadcast addresses, the first and last usable host, the usable and total counts, the wildcard mask used in Cisco access lists, and the address as a 32-bit integer and in hex. A line above the cards tells you the class and whether the address falls in a private, loopback, link-local, CGNAT, multicast or documentation range.
The binary table lays out the address, mask, network and broadcast bit by bit with the network portion shaded, which is the quickest way to see why 192.168.1.130/25 lands in the upper half of the /24. Under "split into smaller subnets" choose a longer prefix to carve the block up: a /24 into four /26s, a /16 into 256 /24s, and so on. The table lists each child network with its usable range and broadcast, and the Download CSV button saves the full list even when the page only displays the first 256 rows. Switch to the IPv6 tab for compressed and expanded forms, the network prefix, the first and last address of the block and the address type.
Why use it
Subnetting mistakes are cheap to make and expensive to find. An access list written with a subnet mask instead of a wildcard, a DHCP scope that starts one address too early, or two VLANs that quietly overlap because someone assumed a /23 ended at .255 can each cost an afternoon of troubleshooting. Checking the boundaries here before committing a change takes seconds and removes the guesswork from the arithmetic.
It is also a practical study aid. If you are preparing for a networking exam, work the example on paper first, then compare your answer with the binary view to see exactly which bit you got wrong. All of the calculation runs in your browser with unsigned 32-bit integer maths for IPv4 and 128-bit BigInt maths for IPv6, so nothing you type is sent anywhere and the page keeps working offline once it has loaded.