Subnet Calculator

Enter a CIDR for IPv4 or IPv6. Calculations happen as you type, locally in your browser.

What is CIDR?

The 30-second version

CIDR (Classless Inter-Domain Routing) is just the format address/prefix. The prefix is how many bits of the address identify the network. The remaining bits identify hosts inside that network.

  • /24 in IPv4 means 24 network bits, 8 host bits, so 256 addresses total (254 usable).
  • /16 means 65,536 addresses.
  • /32 means a single address.
  • /0 means "the entire internet".

IPv4 vs IPv6 vocabulary

In IPv4, the first address in a subnet is the "network" address and the last is the "broadcast" address. Neither is assigned to a host, which is where the "usable" count of 2^host_bits - 2 comes from.

IPv6 has no broadcast concept and no reserved addresses inside a subnet, so every address in the range is usable. The smallest prefix you typically assign to a real subnet is /64 because that's what stateless address autoconfiguration (SLAAC) expects.

Quick reference

  • Private IPv4 ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
  • Loopback: IPv4 127.0.0.0/8, IPv6 ::1/128.
  • Link-local: IPv4 169.254.0.0/16, IPv6 fe80::/10.
  • IPv6 unique-local: fc00::/7 (think "private" for IPv6).