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

Besides those A,B,C intranet classes, there are other reserved addresses not used as public internet IPs, especially:

• 127.x.y.z (loopback)
• 0.x.y.z (reserved broadcast)
• 224-255.x.y.z (multicast & future use)

There are also many wasted IP addresses (legacy from stupid ages). You won't see most addresses in range

• cca 3-40.x.y.z (i.e. 17.x.y.z wasted for Apple Computers)