Recommended read - for practical reasons it is very hard to make a TCP connection to port 0, or to run a client on port 0. Yet, when acidvegas scanned the internet on port 0, they found a lot of interesting stuff:
https://acid.vegas/blog/the-port-that-does-not-exist/
The Port That Does Not Exist
Port 0 is the port you are not supposed to use, so nobody looks there. So we put an IRC network on it. Then I got curious whether anyone else had, and scanned all of IPv4 for port 0. Turns out we weren't alone, and some of our neighbours really shouldn't be there.
acid.vegas