Each "application" connection can use a couple of tcp ports.
Try to check which connections use port 1521, and which anoters ports.
Then OS may keep connection after executing DB command disconnected.

netstat | grep 1521

Some applications open sockets for interact into one box (like UNIX loging)
Just exists BIG difference between network/soket connections and db connections.

[Edited by Shestakov on 08-08-2002 at 01:48 PM]