Question

How can we distinguish "internal" ip addresses from "external"?

Answer

Whether an IP address is internal or not can be identified by the IP range it belongs to. The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets:

10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

Additional Information

More details on this topic (RFC 1918 - "Address Allocation for Private Internets") can be found at: http://www.faqs.org/rfcs/rfc1918.html

• Article last edited: 15-Jan-2014