Free tool
Enter an address with a prefix length. IPv4 gives you mask, broadcast, usable hosts and the reverse zone; IPv6 gives you the range, the number of /64s and the ip6.arpa zone.
What this checks
The number after the slash is how many leading bits identify the network; the rest identify hosts within it. IPv4 /24 is 256 addresses, of which the first (network) and last (broadcast) are not usable on an ordinary subnet, leaving 254; /31 is the point-to-point exception with two usable addresses. IPv6 has no broadcast and no usable-host arithmetic: a /64 is one subnet with 2^64 addresses, and the interesting number is how many /64s a prefix contains. Reverse DNS delegations follow octet boundaries in IPv4 and nibble (4-bit) boundaries in IPv6.
Related tools
Network services
ASN, IPv6 PA and PI sponsored through our RIPE NCC membership, and a fixed public IP over a tunnel for anything behind CGNAT.
