How to Defrag a Tablespace?

I've USER tablespace made of 1 datafile of 100 MByte, 25MByte are free = (auto-extent is off).
When I create a new table with an Initial Extent of 9MByte I get error = ORA-1658: "Failed to find sufficient contiguous space to allocate = INITIAL extent for segment being created." How can I check tablespace fragmentation? How can I reduce tablespace = fragmentation when it's fragmented?

 Luca Minudel                    software designer
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