Distinguish the between intranet and official IP addresses

Question

How can I find out whether a given IP address is an official internet address or an internal intranet IP address?

Answer 1

Intranet IP's are often in the private IP range:

10.0.0.0 through 10.255.255.255
172.16.0.0 through 172.31.255.255
192.168.0.0 through 192.168.255.255

These are not usable for internet IP addresses.

See also http://compnetworking.about.com/od/workingwithipaddresses/f/privateipaddr.htm