Service-Point Addressing

Computers often run several programs at the same time. For example, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process on one computer to a specific process on the other. The transport layer header must therefore include a type of address called service-point address or port address. The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.