libcidr is a library that provides a number of functions to input, output,
manipulate, compare, multilate, and otherwise play with, IP addresses and
netblocks.

It supports both IPv4 and IPv6, and provides sufficiently diverse functions
to be useful for everything from log processes to network client and server
programs. It parses addresses in a wide variety of common formats.  It also
provides a plethora of options for formatting them on the output as well.

It can compare them in various ways and give you some useful statistics
about the netblocks in which they reside.

WWW: http://www.over-yonder.net/~fullermd/projects/libcidr