If I have to connect a 8255 or an LCD or an external EEPROM to a AT89c51,how do I allot the base address?
It seems you are talking about memory mapped I/O intermixed with port mapped I/O

1) why, oh why are you using 8255? There are so many better solutions.
2) base address apply only for memory mapped I/O and the hardware set the base address, so if you write to something memory mapped at say 0ff00h it will be
thang equ 0ff00h
..
..
mov dptr,#thang
mov a,data_for_thang
movx @dptr,a
..

I do not know to which extent I understood your question, if this is not it, please elaborate.

Erik