Most newer Crestron processors (e.g. 3-series and 4-series control systems) support additional ethernet adapters by using a USB-Ethernet adapter, along with applying a software license (SW-USB-ETHERNET). This allows you to have a secondary or tertiary ethernet interface on your Crestron processor, in order to connect to equipment that may be on a private subnet that does not communicate with the primary interface(s).
This is especially helpful in environments where the network is managed by others or where corporate/government network policies require additional LAN separation beyond traditional VLANS.
One ethernet adapter that will work with the 3-series and 4-series processors is the Portsmith PSA1U1E which is conveniently available through Amazon. It provides a low-cost, industrial quality adapter providing the additional functionality for your installation.