Why is OPC Classic considered firewall unfriendly?

OPC Classic uses Microsoft's DCOM (Distributed Component Object Model) for communication, which dynamically opens multiple ports, making it extremely difficult to manage with firewalls.

“OPC Classic is firewall-unfriendly because DCOM requires dynamic port negotiation, making it difficult to define consistent firewall rules.”

— ISA/IEC 62443-3-3:2013, Annex A – Communication Protocols and Security Concerns

This lack of port predictability presents a significant security and operational risk, which led to the development of OPC UA, which uses fixed ports and supports encryption.