port

A number from 0 to 65535 that picks which program on a computer gets a message.

A port is a 16-bit number in the TCP or UDP header that selects the receiving application on a host; servers listen on well-known ports such as 53 (DNS), 80 (HTTP) and 1883 (MQTT).

Examples

The sensor's connection goes from ephemeral port 49152 to the broker's well-known port 1883.
Port 53 is DNS, 80 is HTTP, 443 is HTTPS, 1883 is MQTT and 8883 is MQTT over TLS.