Third-party products can create firewalls between the Internet and your intranet. Most of these products are based on packet filtering. Packet filtering takes place when the computer examines the source and destination IP addresses of a packet and forwards only those packets that have been granted access.
Many third-party routers that connect your intranet to the Internet can be configured to filter packets based on the source or destination IP address. You can specify which IP addresses are allowed or denied access into your intranet.
For more information about the packet filtering products available, consult dedicated router vendors.