We have worked with the default configuration for a Mosquitto server in Linux, macOS, and Windows. The Mosquitto server will work with TCP port 1883. If you want to interact with the Mosquitto server from a different device or computer, you have to make sure that the firewall that is running on your computer has the appropriate configuration for this port number.

When you run a Mosquitto server on a Linux or Windows virtual machine in the cloud, you also have to make sure that the virtual machine network filter that controls both inbound and outbound traffic has the appropriate configuration to allow inbound and outbound traffic on port 1883. You must authorize inbound and outbound traffic on port 1883.