This ARINC Standard specifies the ARINC 758 Mark 2 Communications Management Unit (CMU) as an on-board message router capable of managing various datalink networks and services available to the aircraft. Supplement 4 adds Ethernet interfaces, per ARINC Specification 664 Part 2. This will allow the CMU to communicate with IP based radio transceivers (e.g., L-Band Satellite Communication Systems (Inmarsat SwiftBroadband (SBB) and Iridium Certus), ACARS over IP, AeroMACS, etc.).