Routers are often configured to have a capability commonly referred to as proxy ARP. Proxy ARP responds to machines in one subnet that are trying to resolve addresses that are outside of the range of addresses in its own subnet (e.g., a host within another subnet). The router will provide the IP address/Media Access Control (MAC) address association for the router interface address as a response to such requests. As known in computer networking, a MAC address is a unique identifier assigned to most network adapters or network interface cards (NICs) by the manufacturer for identification, and used in the MAC protocol sub-layer. Proxy ARP can help a machine on a subnet reach a machine on a remote subnet without the need to reconfigure routing or provide a default gateway in the network.
Certain plants (e.g., petrochemical plants) include embedded process controllers that utilize secure networks to avoid the embedded process controllers from being manipulated by unauthorized individuals. In such arrangements, the router's proxy ARP is disabled and the embedded processor (s) utilize a separate IP address range, or subnet, that is not in the main plant subnet, and is not contactable through the plant's bridge or router. Static routes are added to embedded controllers and one or more machines in the main plant subnet (e.g., a server) to enable the machines in one subnet to know how to reach machines in the other subnet.