Had to read quite a bit before getting to the core insight:

> “So when you listen on a port, you’re really listening on a combination of a port, an IP, a protocol, and an IP version?”

> “Yeah, unless you listen on all local IPs. And if you listen on all IPv6 IPs, you also listen on all IPv4 IPs, unless you specifically ask not to before you call bind.”

> “Right. So the operating system must have, like, a hash map from a port and IP pair to a socket, for each combination of TCP or UDP, IPv4 or IPv6.”*

> “To a list of sockets”, Liz corrects. “Remember how I could listen on more than one?”

> “But it also has to handle listening on all ‘home’ IPs, and to be able to find a socket listening on IPv6 from an IPv4 IP.”