How to apply filters and mask on a MCP2515

Hi, I'm working with the Sparkfun CANbus shield with the MCP2515 can chip. I'm a bit puzzled about masks and filters using extended CAN. In the MCP 2515 data sheet it states that Masks and filter is applicable on all 29bit of the identifier, and not only on those 18 bits building the PGN. On the other side, in the data sheet, there is only, for booth mask and filter anly 18 bits available. Can anyone explain this to me, and also, what bit is bit 0 in the message ID when applying the mask/filter.

Thanks' _/) P-L