Insufficient information.
What version OS are you running?
If you are running one of the recent Debian releases, I2C-2, (in early software was called I2C-1) which is the one that comes out to detect other capes, on P9-17 and P9-18 is already enabled, and ready to use.
There are four (EEPROM memory) addresses that are reserved, but everything else is available to you to use.
What are you trying to do?
--- Graham
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