Calculate a subnet.

What does /24 mean?

The number after the slash is the prefix: the number of bits that identify the network. The rest is for the devices in that network. With /24, 24 of the 32 bits belong to the network and 8 remain, enough for 256 addresses. Two of those are taken (network and broadcast), which leaves 254 devices.

What is the difference between a prefix and a subnet mask?

It is the same information, written differently. /24 is the same as 255.255.255.0, and /16 is 255.255.0.0. You can enter either one here.

Why can I not use the first and last address?

In an IPv4 subnet the first address is the name of the network itself and the last one is the broadcast address, which reaches all devices at once. The exceptions are /31 (two addresses, for a link between two routers) and /32 (one address). IPv6 has no broadcast.