How to configure port for listening to traffic meant for PostgreSQL
This is basic TCP/IP, rather than PostgreSQL-specific.

If the only things that will talk directly to the database server are on the same host as the database server, you only need listen_addresses = 127.0.0.1, i.e. listen only for local connections. For example, if you have pgbouncer proxying connections and accepting connections on a different port. Or if all user interaction with the DB happens via an application server and webapp.



The address 127.0.0.1 is by convention reserved for the local host. It's for talking to yourself.

There is no practical reason to allow (or block) connections to 127.0.0.1 from outside hosts, because no router will ever send packets to your host with that destination address, and your machine would never respond to ARP requests for that address. Most operating systems would ignore packets destined for 127.0.0.1 coming from external network interfaces as obviously bogus.

So you want to open connections from other hosts. 0.0.0.0/0 on port 5432 is "let anything connect to port 5432". That's an option if you don't want to be more restrictive about who can talk to the server.

Note that you may have to adjust both firewall rules and PostgreSQL's listen_addresses directive. Also possibly pg_hba.conf.

listen_addresses controls which local network interfaces PostgreSQL will accept connection attempts from.

pg_hba.conf controls which remote hosts PostgreSQL will accept connection hosts from, assuming it's even listening.

Firewall rules will stop those connection attempts even reaching PostgreSQL by filtering based on source address, destination port, and other criteria.