IPv4 subnet calculator (CIDR)

How do you calculate a subnet from CIDR notation?

The prefix after the slash says how many leading bits of the address name the network; the remaining bits number the hosts. A /24 keeps 24 network bits and leaves 8 host bits, so it holds 2⁸ = 256 addresses, and its netmask is 24 one-bits followed by 8 zero-bits: 255.255.255.0.

The network address is the address with every host bit set to 0, and the broadcast address has every host bit set to 1. Neither can be given to a host, so a subnet has 2 fewer usable addresses than its size: 254 in a /24 and 2 in a /30. A /31 is the exception, because RFC 3021 lets a point-to-point link use both of its addresses, and a /32 is one single address.

The wildcard mask is the netmask with every bit flipped, 0.0.0.255 for a /24; router access lists use it to mark the bits that may vary. The calculator also says whether the address sits in a range set aside for a special purpose, such as private-use 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 from RFC 1918, or 100.64.0.0/10 for carrier-grade NAT; anything outside those ranges is labelled public.

addresses = 2^(32 − prefix)
usable hosts = addresses − 2   (/31 = 2, /32 = 1)
network = address AND netmask
broadcast = network OR wildcard mask