Enter an address in CIDR notation like 192.168.1.0/24 into a browser-based subnet calculator and it works out the network address, broadcast address, usable host range and the number of hosts. It runs on your device, turning subnet-mask arithmetic into a set of numbers you can read at a glance.
CIDR notation writes an address followed by a slash and a prefix length, the number of leading bits that identify the network. A /24 means the first 24 bits are the network and the last 8 identify hosts, which gives 256 addresses, 254 of them usable once you set aside the network and broadcast addresses. A calculator derives the mask, the ranges and the host count directly from that prefix.
Subnetting by hand means converting between decimal and binary, applying the mask, and being careful with off-by-one errors on the usable range. The calculator removes all of that: you get the network and broadcast addresses, the first and last usable host, and the total, instantly. That is exactly what you need when planning an IP range, configuring a router, or checking whether two addresses share a subnet.
254. A /24 has 256 addresses, minus the network and broadcast addresses.
It is the prefix length, the count of leading bits that identify the network. A larger number means a smaller subnet.
No. The calculation runs entirely in your browser.