To check what connections to ports are open on your computer:
ss -tlnp

Output:

State    Recv-Q   Send-Q     Local Address:Port      Peer Address:Port                                                                                        
LISTEN   0        128              0.0.0.0:22             0.0.0.0:*      users:(("sshd",pid=445,fd=3))                                                        
LISTEN   0        100              0.0.0.0:25             0.0.0.0:*      users:(("master",pid=929,fd=13))                                                     
LISTEN   0        128                    *:3306                 *:*      users:(("mysqld",pid=534,fd=30))                                                     
LISTEN   0        128                    *:80                   *:*      users:(("apache2",pid=765,fd=4),("apache2",pid=764,fd=4),("apache2",pid=515,fd=4))   
LISTEN   0        128                 [::]:22                [::]:*      users:(("sshd",pid=445,fd=4))                                                        
LISTEN   0        100                 [::]:25                [::]:*      users:(("master",pid=929,fd=14))                                                     
LISTEN   0        70                     *:33060                *:*      users:(("mysqld",pid=534,fd=33))