<...scratching head...>
For programming purposes, original ECOMs spoke only when spoken to and replied to the address that spoke to them. If that was off of the local network, the ECOM would hear from, and then reply to, the gateway. No setup was necessary. I am a little fuzzy on the peer to peer connection, but I don't think it would cross routers. Since the original ECOM didn't generate its own traffic, a netmask and gateway wasn't necessary.
The ECOM100 can generate its own traffic (email, for instance) and therefore does require those to be configured.
It's a bit fuzzy, but I seem to remember that the ECOM had a gateway field, but I don't think it did what you think it did.