What is a "private IP address range"?

Q. What is a "private IP address range"?

A. The Internet Assigned Numbers Authority (IANA) has reserved several blocks of IP addresses that an organization may assign for its own private internet. These blocks are defined in RFC 1918, http://tools.ietf.org/html/rfc1918.
From the RFC:

Private Address Space: 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 (10.0.0.0/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16.0.0/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168.0.0/16 prefix)