When it is possible, users of unregistered (or "dirty") networks should use the reserved addresses in RFC 1918 on any networks inside the PIX.
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This document is not restricted to specific software and hardware versions.
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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.
You can grow your internal IP networks without worrying about running out of addresses.
You can eliminate the risk of inadvertently using other networks' legitimate addresses.
For example, if you use the Class C range of 188.8.131.52 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 184.108.40.206). This is because your hosts try to reach a machine that does not exist on the inside of your firewall.
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