CIDR/Subnet Calculator
IPv4 and IPv6 subnet calculator with RFC 3021 handling for /31 point-to-point ranges, explicit start-end IP range analysis, IPv6 range and split planning, optional IPv4 split planning, IPv4 VLSM host-group allocation, IPv4 free-space planning from pasted used subnets or hosts, and overlap or aggregate planning for pasted IPv4 or IPv6 hosts and CIDRs.