How about an "open" port with nothing listening on it? What does "open" mean?

I think his point is that two nodes should be able to open and then connect on any port they choose, not just TCP 80. Please correct me if I'm wrong.

Are we saying that a user should not even be _allowed_ to have an application listen on any port except a known few?

Isn't the more important issue whether an application/process is actually listening on a port? I mean you can send me all the nasty bits you want but unless I'm prepared to receive them, what difference does it make? And I thought the whole premise here is that the only people on this social network are ones you know. They shouldn't be deliberately sending you nasty bits anyway.

I am a unixnoob. Please go easy on me for my ignorance.

I think it is true that if nothing is listening on the port, then the packets will be dropped by the OS. However, you may have services you are unaware of running, or malicious software which has snuck in, or a running application which would be listening on the port if it was open. Perhaps you want to run a webserver on localhost for testing, and by keeping the ports closed you block remote access. For these reasons it is generally regarded as bad practice to leave all ports on a server open to the public.