Donaldson R F Subnet / CIDR Calculator
Calculate network details from an IP address and CIDR prefix — IPv4 or IPv6.
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IPv6 notes
For IPv6, "Network Address" and "Last Address" mark the first and last address in the block (IPv6 has no broadcast address, so there's no separate broadcast/usable-host concept the way IPv4 has it). Addresses are shown in their compressed form (the longest run of consecutive all-zero groups replaced by ::, per RFC 5952) and total-address counts can be astronomically large for short prefixes, so they're shown as a plain number rather than rounded.
Looking for an IPv6 shortening/expanding tool on its own? This calculator's compression follows the same rule a dedicated one would use.