postgresql definitive list of network resources used/needed?

Apparently, I've discovered that PostgreSQL connects to itself over UDP.  I've also discovered, while looking at the network while one of our server was running PostgreSQL, that while there were a number of connections between local 5432 and a random remote port, there was also a connection from a local random port to a remote 5432.  If these ports were closed and only 5432 left open, this connection would have failed.

Thanks, 

rik.