Excellent description. I often use the metaphor of an apartment building's mailboxes with "the internet" (really, the network elements, obv) being the postal service, the building being the IP address and the mailboxes being the ports, mapped to services/programs. So far, it's worked pretty well. This also lets me extend the discussion to TCP vs UDP, where I compare UDP to simply mailing a letter and TCP to requiring a signature verifying receipt.