IP addresses in the range: - 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)
Machines with these addresses do not have direct access to the Internet (they cannot "ping"). In order to access Internet resources, they must go through a proxy firewall.
Furthermore, there are other networking addresses that should never be configured
0.0.0.0 The address "0.0.0.0" used mean the same as "255.255.255.255", which is the "local broadcast". Also, many systems use this address to mean "no address". For example, several intrusion detection systems use this address in the cases of IP spoofing where the attacking address isn't known. 0.x.x.x Many protocol stacks use this address range for built-in properties. It should never be used external to a machine. 127.0.0.1 This is the loopback address. The command "ping 127.0.0.1" is generally used to see if the TCP/IP stack works at all. This just causes the machine to ping itself. 127.x.x.x Actually, any 127.x.x.x address can be used to refer to the loopback address. 192.0.2.x This address range is earmarked for written examples. Books and documentation that refers to IP addresses should use this range. It is similar to the phone number 555-XXXX seen in movies. In the movie "The Net" staring Sandra Bullock, the IP address of "23.75.345.200" is shown in much the same context. Of course, it is impossible because "345" exceeds an 8-bit value (max=255). They should have used a 192.0.2.x address instead.
In much the same way, the DNS names "example.net" and "example.com" have been reserved for documentation purposes.
169.254.x.x This address range has been reserved for use when DHCP configuration fails. Increasingly, routers on the Internet are being assigned "private" addresses. These addresses are used for communication with the router itself. As a result, packets sent by the router itself will use these private address (example: traceroute, Path MTU discovery). Many filters block these private addresses, resulting in these connections being dropped.
