What system is it 115U or 135/155U?
If it is 115U then does it use an IM306 module, if so then addresses are set by DIP switch on the IM306, otherwise addresses are set by module position in rack, apart from IP modules which have the address set by a DIP switch on the module.
If it is 135U/155U then all addresses are set by means of DIP switches on the modules. Also if you have a really big system you may use the OW addresses as well as the PW addresses, this is set at a rack level. Only exception that comes to mind is IM308C profibus interface where all addresses are set by the configuration file held on the memory module.