This read-only MacAddress, held on the ComController, identifies which ComPort a row in the EtherType filter table belongs to, working with the accompanying stuFiltEtherType and stuFiltEtherStatus attributes for that entry. An admin uses this to locate the correct STU's EtherType permit-list before adding or removing an allowed protocol type for that specific subscriber. For example, before adding a new permitted EtherType entry for a customer running a specialized protocol, an operator confirms stuFiltEtherMacAddr matches that customer's STU.