Logon to the DB2 system as the DB2 user and run the following command:
db2 get dbm cfg | grep -i svcename
This will give you output like the following:
TCP/IP Service name (SVCENAME) = db2c_cdbinst1
Take the SVCENAME value and grep for it in the /etc/services file.
grep db2c_cdbinst1 /etc/services
This will give you the port.
In this case 50005.