RFC 1918 - non-routable private IP Address Ranges

RFC 1918 private and non-routable IP address ranges are:

24-bit block	10.0.0.0 – 10.255.255.255	8 bits
20-bit block	172.16.0.0 – 172.31.255.255	12 bits
16-bit block	192.168.0.0 – 192.168.255.255	16 bits

The above are IP addresses which can not be scanned or reached from outside an organisation for pen testing etc.