Because a whole suite of protocols had to be invented: * https://en.wikipedia.org/wiki/STUN https://en.wikipedia.org/wiki/STUN * https://en.wikipedia.org/wiki

Because a whole suite of protocols had to be invented: * https://en.wikipedia.org/wiki/STUN https://en.wikipedia.org/wiki/STUN * https://en.wikipedia.org/wiki/Traversal_Using_Relays_around_NAT https://en.wikipedia.org/wiki/Traversal_Using_Relays_around_... * https://en.wikipedia.org/wiki/Interactive_Connectivity_Establishment https://en.wikipedia.org/wiki/Interactive_Connectivity_Estab... See also "NAT Issues when playing games": * https://support.plume.com/hc/en-us/articles/360035745134-NAT-Issues-when-playing-games- https://support.plume.com/hc/en-us/articles/360035745134-NAT... Certainly we'd need hole punching protocols (UPnP/PCP) because IPv6 firewalls would be default deny on incoming connections, but the addressing issue introduces a bunch of things on top of that. The fact that these kludges are considered 'normal' and/or 'okay' is kind of sad. And how do you hole punch through CGNAT? (AFAIK there is no way.) Everything is being crammed into HTTP/QUIC/whatever because of all of these limitations: what applications could have been designed better if bits flowed more freely? (SCTP and DCCP died because of limited middleware network boxes.)