I was using a port probing site/program called ShieldsUp! to scan my ports. It reported back that all of my ports were "stealth", and in their words
Your system has achieved a perfect "TruStealth" rating. Not a single packet — solicited or otherwise — was received from your system as a result of our security probing tests. Your system ignored and refused to reply to repeated Pings (ICMP Echo Requests). From the standpoint of the passing probes of any hacker, this machine does not exist on the Internet. Some questionable personal security systems expose their users by attempting to "counter-probe the prober", thus revealing themselves. But your system wisely remained silent in every way. Very nice.
Then I ran the same port scan when I knew that I had a program that was using certain ports and the test came back stealth again.
Is that to be expected? I was under the impression that using the ports made them visible. I was expecting it to find that many of my ports were simply closed based on their Internet Port Status Definition because I was running these tests on a fresh installation of Ubuntu with no third party firewall installed.
But even if Ubuntu had a built in firewall, I had to tell it to specifically open the ports I wanted to use. Would Ubuntu automatically close those ports when it didn't get any traffic coming or going?
Additional information (wasn't sure it would matter at first)
I left out part of the explanation because I wasn't sure it would be relevant, but now I think it might be. I had set it up so that two computers on my network were communicating using a simple communication client. And while the two computers were communicating, I ran the port probe on one of the computers. I only mention this because it seems like their would be ports open because each computer would be expecting traffic on a given port.