Sounds like you're describing NAT-PT, originally proposed in RFC 2766 (published in 2000). It suffers from a lot of practical problems, which are described at

Sounds like you're describing NAT-PT, originally proposed in RFC 2766 (published in 2000). It suffers from a lot of practical problems, which are described at length in RFC 4966. For example, without careful special handling it breaks protocols like FTP/SNMP/H.323 that embed IP addresses in data packets; it breaks IPsec; it can't handle fragmentation; and it suffers from scalability issues. https://datatracker.ietf.org/doc/html/rfc2766 https://datatracker.ietf.org/doc/html/rfc2766 https://datatracker.ietf.org/doc/html/rfc4966 https://datatracker.ietf.org/doc/html/rfc4966