For UDP unicast, the address information for the Subscriber is configured on the PubSubConnection and the address information for the Publisher is configured on the WriterGroup.

The receive port for UDP unicast communication is configured on a PubSubConnection. The Address parameter for a PubSubConnection is defined in 6.2.7.3. All NetworkMessages for one port are received through one PubSubConnection. The filtering and assignment of NetworkMessages for the Subscriber is done based on the PublisherId. The hostname for the Url in the PubSubConnection Address parameter is set to ‘localhost’ since the source address is not used for filtering. The NetworkInterface field in the Address is not required and is only configured if the Subscriber should listen only on the configured interface. If the NetworkInterface is null or empty, the Subscriber should listen on all interfaces.

The syntax of the Url field in the PubSubConnection Address parameter has the following form:

opc.udp://localhost[:<port>]

The destination address is configured on the datagram TransportSettings of a WriterGroup. The Address parameter for a WriterGroup datagram TransportSetting is defined in 6.4.1.3.4. The Address parameter for WriterGroup datagram TransportSettings shall be configured. If no Address is configured on a WriterGroup, the WriterGroup PubSubState shall be Error. The NetworkInterface field in the Address is not required and should be null or empty and shall be ignored.

The syntax of Url field in the WriterGroup datagram TransportSettings Address parameter has the following form:

opc.udp://<host>[:<port>]

The host is either an unicast IP address or a registered name like a hostname or domain name that can be resolved to a unicast IP addresses. The IP address and the port are the destination of the UDP datagram.

The IANA registered OPC UA port for UDP communication is 4840. This is the default and recommended port for unicast communication. Alternative ports may be used.