The subcalc utility is used for subnet calculation and IPv6 DNS PTR
record generation.  subcalc takes command line arguments in a similar
format to ifconfig(8) so the synopsis should be familiar to the user.
Given an address family, address and a netmask/prefix length, subcalc
will calculate the number of hosts and address ranges of the specific
network.

WWW: http://www.seccuris.com