Reading CIDR notation
The /N suffix says how many leading bits of the address identify the network. /24 leaves 8 bits for hosts (256 addresses, 254 usable), /16 leaves 65,536 addresses, and /30 leaves just 4 — a common size for point-to-point links.
Network vs broadcast
The first address in a subnet is the network identifier and the last is the broadcast address; neither can be assigned to a host. Everything between them is the usable range this calculator reports.