Port 1521 is used for ages as the default port for the Oracle listener, but while I was looking at some default port numbers on Wikipedia, I saw port 1521 is not the official listener port. 1521 is the nCube License Manager official port!
Port 2483 and 2484 are the official Oracle ports according to IANA!
ttc 2483 tcp Oracle TTC
ttc 2483 udp Oracle TTC
ttc-ssl 2484 tcp Oracle TTC SSL
ttc-ssl 2484 udp Oracle TTC SSL
According to Metalink’s document “Listening Port numbers [ID 99721.1]“, in Future releases the 1521 port may change to 2483 TCP and 2484 TCP with SSL, but this is document is written in 2008…
Edit: The metalink note has been removed…