grepcidr can be used to filter a list of IP addresses against one
or more Classless Inter-Domain Routing (CIDR) specifications, or
arbitrary networks specified by an address range. As with grep,
there are options to invert matching and load patterns from a file.
grepcidr is capable of comparing thousands or even millions of IPs
to networks with little memory usage and in reasonable computation
time.

grepcidr has endless uses in network software, including: mail
filtering and processing, network security, log analysis, and many
custom applications.

WWW: http://www.pc-tools.net/unix/grepcidr/