there is NO DEFAULT hostring.( some seed databases carry some demo hoststring with em).
edit your tnsnames.ora and add entries to create a hoststring.
If you u have multiple databases in  multiple home,
you can have 
either
   single listener for all databases
or
   single listener for each database.
IF this is prod environmnet in DMZ etc
its better to have ONE LISTENER PER DATABASE.
else
you can just have one listener servicing multiple databases (which is easy, in administration point of view).
for multiple listeners 
1. OPEN different ports.
2. configure listeners to listen to their respective ports and add entries.