zlacker

Without bidirectional NAT, hole-punching works. Two sides of a p2p connection can coordinate with an intermediary to learn each other's addresses. They send each other a packet, which gets dropped by the other side. Their firewall sees the outgoing packet though, and opens the port. The next time they send each other packets, they will be allowed through. The intermediary is only needed to do the initial handshake instead of for all packets.

With NAT, it doesn't work because the ports get remapped, and the intermediary doesn't know how they will get remapped on the p2p connection, so they can't coordinate to send on the correct ports to open the firewall.

Or UPnP can work. By default, your router drops incoming packets on all ports. If you want to e.g. run a game server, then on startup, it hits a standard API to tell the router to forward that one port. On shutdown, it can tell it to close the port (you could potentially also have the router require keepalives to keep the forward alive. I'm not familiar with the details of UPnP and related protocols).

Without a public IP, you need intermediate servers to relay all traffic to you, which centralizes the web. With p2p working, you can e.g. have high quality video calls with friends/family instead of dealing with the garbage quality tech companies allow. Or I can share with my mom photos of her grandkids with effectively unlimited storage; for 2 years of 2 TB Google storage, I can buy 20 TB of disks.