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Which TCP port is used by DCOM for OPC communication?

Issue
If DCOM is used to enable OPC communication among computers, which TCP port needs to be opened through a firewall?

Product Line
ION OPC Server, 3rd party OPC Client, ION OPC Test Client

Environment
Internal network of a group of computers, LAN

Cause
DCOM (Distributed Component Object Model) is a Microsoft technology for establishing communication among applications running on different computers. Historically, OPC relied on DCOM for communicating among computers. For example, a computer running OPC client, trying to connect an OPC server on a different computer would rely on DCOM for the communication. The DCOM needs a particular port to be opened in the firewall, in both client and server machine, to have the communication succeed.

Resolution
Please open TCP port 135, both on client and server computers, in the firewall, to enable DCOM-based OPC communication to take place.