ISIS NET address format

Hi all

I am preparing the routing part of CCIE, and I am actually running (at least trying) the ISIS part.

Can somebody recall me the NET adress format.

I know that there is an AREA part, a SYSTEM ID and a Selector part, but how are these part delimited.

I mean, if we assume that the NET address can be between 8 and 20 bytes, how is it possible to tell that system ID size is 4, 8 or 16 bytes for example ?

Thanks