When it is possible, users of unregistered (or "dirty") networks should use the reserved addresses in RFC 1918 on any networks inside the PIX.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware versions.
For more information on document conventions, refer to the Cisco Technical Tips Conventions.
As discussed in RFC 1918, the addresses that you should use are shown here.
Class A: 10.0.0.0 - 10.255.255.255 (10/8 prefix)
Class B: 172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
Class C: 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
There are two advantages of using these numbers on the inside of the PIX.
For example, if you use the Class C range of 184.108.40.206 for network addresses on the inside of your PIX, your computers are unable to connect to another machine that has a legitimate IP address (such as 220.127.116.11). This is because your hosts try to reach a machine that does not exist on the inside of your firewall.