Comment by tbrownaw

> IPv4 is from the era of local computer networks, which feature clients and servers. Clients talk to servers, but servers are not supposed to care or even know about clients unless clients decide to reach out to them. Client-to-Client communication is generally discouraged.

No, it was meant to be a global address space where anything could talk to anything. That became unworkable due to scale and the limit inherent in using only 32 bits for the address space.

Some older protocols (ftp) don't play nice with NAT and need special handling, because address multiplexing was never intended to be a thing.