@JeremyCherfas IP address: Somewhere you can deliver mail.
MAC: A specific mail slot.
NAT: Things are sent to Jim @ Huge University. Huge University's mailroom translates that into an internal mailstop and forwards it along internally. When Jim sends mail, it gets sent as from Jim @ Huge University, and responses go back there. Jim's mailstop is known only to the Uni.
// @phoneboy @streakmachine @kdfrawg