How a subnet is calculated
An IPv4 address has 32 bits. The prefix after the slash says how many leading bits identify the network. In 192.168.1.10/24 the first 24 bits are the network and the last 8 are for hosts. The network address sets all host bits to 0, the broadcast address sets them all to 1 and the usable hosts sit in between.
Usable hosts
- For most prefixes: 2(32 - prefix) minus 2 (network and broadcast). A /24 has 254 usable hosts.
- /31 has 2 usable addresses with no broadcast, for point-to-point links (RFC 3021).
- /32 is a single host.
Private and special ranges
The calculator labels common ranges: private addresses (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), carrier-grade NAT (100.64.0.0/10), loopback, link-local, documentation blocks, multicast and reserved space. Everything else is shown as public.
IPv6 is not supported yet. Enter a mask instead of a prefix, as in 192.168.0.5 255.255.255.128, and it is converted for you.