Sipcalc is an ip subnet calculator. 
Features:

IPv4 - 
* Retrieving of address information from interfaces.
* Classful and CIDR output.
* Multiple address and netmask formats (dotted quad, hex, number of bits).
* Output of broadcast address, network class, Cisco wildcard, hosts/range, 
  network range.
* Output of a userdefined number of extra networks.
* The ability to "split" a network based on a smaller netmask,
  now also with recursive runs on the generated subnets.
* DNS resolution. 

IPv6 - 
* Compressed and expanded input addresses.
* Compressed and expanded output.
* Standard IPv6 network output.
* Reverse dns address generation.
* The ability to "split" a network based on a smaller netmask,
  now also with recursive runs on the generated subnets.
* DNS resolution. 

WWW: http://www.routemeister.net/projects/sipcalc/