subnets

http://www.inout.gr/showthread.php?t=19690

http://en.wikipedia.org/wiki/Subnetwork

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f33.shtml

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

http://www.joshgentry.com/networking/subnet.htm

http://www.networkclue.com/routing/tcpip/calculating-masks.aspx