If it's possible to get your hands on an offline copy of the RSLogix500 program, the IP addy of the SLC5/05 may be available in there. As long as the network system is static.
Find and open the processor channel configuration in the project tree. From the channel configuration window, find channel 1, which is the 5/05's Ethernet port. Pertinent network info is viewable and editable there.
Pic attached for clarity.