If you have a lot of networked DL PLCs, then I guarantee that eventually you will program the wrong PLC if you don't use passwords. Every PLC should have a simple unique password for verification, even if you don't think you need it for security. This won't tell you which PLC you are trying to talk to if you don't know, but it will verify that you have the right one.
On the other hand, reading data is simple, since you insert the program and installation identification yourself into memory as you find necessary.
If the ECOM100s are using DHCP, then looking at your router DHCP tables will tell you what addresses are in play for ECOM100s, since ECOM MAC ranges are identifiable.
You can also access the ECOM100 using HTTP to determine it's MAC address.
Note that ALL Ethernet connected hardware has a instance unique MAC address, it's part of the requirement.