Filter IP addresses matching IPv4 CIDR/network specification

 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.