Siemens PLC S400 Addresses 2 and 3 reserved?

Hi there, I have a Step 7 PLC S400, I know Slots 2 and 3 are reserved making the first I/O slot 4, however, when I assign a module to slot 4 the address defaults to I0 and I1, I de-select the DEFAULT ADDRESS check mark and change to I2 and I3 and everything works. Today I have the first input module at I0 and I1 and the second at I2 and I3 everything works, but a co-worker is telling me I can’t use I2 and I3 it is reserved, I think he is confusing Slots 2 and 3, not addresses. Then I see the the default is I0 and I1 for the first slot, then I4 and I5 on the second slot??? why not 2 and 3

Are they really “reserved”? and why does 2 and 3 work?

Also, can I get a better description of the ethernet ports P1 and P2, is there an internal switch,? either port connects automatically to my lap top.


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