Ask someone who is a DBA for that remote database what the service name or port number is for that DB2 instance. This information can be found by logging on to the database server as the instance owner and typing the following command:
db2 get dbm cfg
TCP/IP Service name (SVCENAME) = 50000
If it is a number, then it is a port number, or if alphanumeric it is a service name resolved in /etc/services (or windows equivilent for windows based servers).
The default port number is 50000 in most cases (if you can't get someone to help you and want to guess).
If you cannot log on to the server as instance owner, try looking in the /etc/services file for the port number (should be near the bottom).