RFC 1597 provides for a group of Internet Networks that will never be assigned. Furthermore, these addresses will not route through the Internet. This makes them the proper choice for use in your home or company Intranet. We recommend non-routable IP for most networks. These IP addresses have specifically been set aside to use when it isn't necessary (or desirable) for anyone on the Internet to be able to instantly navigate to your computer. It is especially useful to the DSL and Cable Modem users who have multiple computers connected to a single proxy server, firewall or router.

These non-routable IP addresses also provide additional security on the “Internal” side of your network. Typically a firewall, Cable Modem or DSL router will provide Network Address Translation (NAT) to translate the these nonroutable address to a real Internet routable IP address.

Reserved IP Addresses for Private Networks

10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

For more information on RFC 1597 click here: http://www.faqs.org/rfcs/rfc1597.html