What is a nonroutable IP address?

Answer

One that starts with 192.168. or 10.27.

These addresses are used for a LAN that has access to the internet through a local router which routes traffic to/from any PC connected to it via an address in either of those ranges from/to the internet. Addresses in those ranges are not routed by the internet routers. Most cheap off the shelf routers you can buy at Circuit City or your friendly local computer shop default to 192.168.0. or 192.168.1. as the first three parts of the 4 part IP addresses that they assign to any PCs connected to them. The router then gets a legitimate routable address from whatever ISP supplied router that it connects to for your internet connectivity.

Private Address Space

The Internet Assigned Numbers Authority (IANA) has reserved the

following three blocks of the IP address space 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

In other words 10.123.123.123 is a private non-routable

address for use in your LAN whereas 172.15.255.42 or 172.123.123.123 is not.