I'd start sniffing on the network. Do you have the possibility to sniff on that network (using a tool like WireShark)? You have to get the difference between the telnet on your host and the request from the Arduino. Smart switches usually allow to define a monitoring port where you get all the traffic.
If you cannot do that, describe how your network is constructed (every device connected, like your PC, router, switch, NAS, etc.).