It depends, the port number is set by the user when the Index Agent is installed. It also depends on if you are talking about FAST or xPlore.
I believe the default port is 9081
for the FAST index agents using your url above.
For xPlore, the default configuration url is http://<HOST_NAME>:9200/IndexAgent
.
You may want to take a look here if that doesn't help you: https://community.emc.com/community/support/documentum