Arpwatch

"Arpwatch is a tool that monitors ethernet activity and keeps a database of ethernet/ip address pairings."

(http://www.freebsd.org/cgi/url.cgi?ports/net-mgmt/arpwatch/pkg-descr)

CIDR

CIDR, short for "Classless Inter-Domain Routing", is a new addressing scheme for the Internet which allows for more efficient allocation of IP addresses than the old class based address scheme. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP network prefix.

See RFC 1519