One of the biggest challenges to learning and understanding NAT is the terms that are used to describe the different concepts and configurations. The terms that cause the most confusion are associated with the addressing: inside, outside, local, and global. More than one of these terms can apply to an address, depending on your perspective:
Inside and local refer to the same side of the NAT device, typically called the private or internal network. Outside and global refer to the other side of the NAT device, known as the public or external network. The four possible combinations are described in Table 5.1.
RFC 1918 sets aside network ranges to be used for private networks:
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