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Until version 3.6.0, all versions of netCDF employed only one binary data format, now referred to as netCDF classic format. NetCDF classic is the default format for all versions of netCDF.
In version 3.6.0 a new binary format was introduced, 64-bit offset format. Nearly identical to netCDF classic format, it uses 64-bit offsets (hence the name), and allows users to create far larger datasets.
In version 4.0.0 a third binary format was introduced: the HDF5 format. Starting with this version, the netCDF library can use HDF5 files as it's base format. (Only HDF5 files created with netCDF-4 can be understood by netCDF-4).
By default, netCDF uses the classic format. To use the 64-bit offset or netCDF-4/HDF5 format, set the appropriate constant when creating the file.
To achieve network-transparency (machine-independence), netCDF classic and 64-bit offset formats are implemented in terms of an external representation much like XDR (eXternal Data Representation, see http://www.ietf.org/rfc/rfc1832.txt), a standard for describing and encoding data. This representation provides encoding of data into machine-independent sequences of bits. It has been implemented on a wide variety of computers, by assuming only that eight-bit bytes can be encoded and decoded in a consistent way. The IEEE 754 floating-point standard is used for floating-point data representation.
Descriptions of the overall structure of netCDF classic and 64-bit offset files are provided later in this manual. See Structure.
The details of the classic and 64-bit offset formats are described in an appendix. See File Format. However, users are discouraged from using the format specification to develop independent low-level software for reading and writing netCDF files, because this could lead to compatibility problems if the format is ever modified.
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