Hi,
To obtain a list of all the open ports of your system and which are the associated applications to them just execute command:
netstat -tulp ip
or nmap -sT -O ip
9 may 2022 update:
As @rogierm points out, netstat is obsolete and its replacement is ss.
Then the equivalent command to with ss would be:
$ sudo ss -tulp
You can read more about this topic here:
difference between netstat and ss in linux?