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

RFC 1918 defines some IP address ranges which can't exist on the internet, and are therefore suitable for intranet use (i.e. 10/8, 172.16/12, 192.168/16).

Apart from that, what do you mean? Are you trying to find out which IPs are on the same subnet as your own machine, or on some other 3rd party network?

See also http://en.wikipedia.org/wiki/Private_network