Q: What process is listening on port P?

You can get this info from the lsof(1) program:

   lsof -i :234

gives you a list of the process(es) listening on port 234.  Note  the
':' before the port number.

One problem is that for interpreted program in languages  like  java,
perl  and  python,  you  get the name of the interpreter, which isn't
very informative.  You may need to use the ps(1) program to  discover
the name of the program:

   ps -fp 1234

where 1234 is the process number.

If you have netstat installed, it can also do the job:

   sudo netstat -peant | grep ":80 "

The space in the quoted string is useful to exclude  processes  using
longer port numbers like 800 or 807.