Why can a host on a private range start a conversation with a public server, while a public host cannot start one with it?
Answer
The gateway holds a public address and translates on the private network’s behalf, so there is no return path until the inside starts the exchange
Routers on the public internet drop private addresses, so a packet addressed to one has nowhere to go. Outbound works because the gateway substitutes its own public address and keeps enough state to send the reply back. The asymmetry is a consequence of that translation, not a property of the addresses themselves — they are ordinary IPv4 addresses usable in either direction inside the network. It is not an ISP policy that can be lifted, since the whole point of the reserved ranges is that everyone reuses them. And there is no separate protocol number involved; the packets are the same IPv4 packets.