Addressing inputs/outputs in SLC500

I was looking for some help with Allen Bradley's I/O addressing scheme for the SLC-500.

I'd like to know how the address format when multiple racks are being used. I already know the format for single-rack systems,

ie:

I:x.y/z (I could be O in the case of an output card)

where:

x=slot number
y=word number (I often omit this because I rarely use more than 16 inputs at school)
z=bit (terminal) number

I've looked at AB's Addressing Reference, but multiple rack addressing isn't explained.

Thanks in advance,

Richard Byrne

Hope this helps