Which of the following best describes multiplexing in transport protocols?

• A. IP addresses
• B. MAC addresses
• C. Port numbers
• D. VLAN tags

Answer: (C)

Multiplexing at the transport layer means letting several applications share the same network path by tagging each data stream with its own port number. Each transport segment carries a source and destination port number, and these, along with the IP addresses, guide the data to the correct application on the destination host. When data arrives, the system uses those port numbers to deliver the bytes to the right socket, effectively separating the many streams that traveled over the same connection. IP addresses identify the two hosts involved, but they don’t distinguish between different applications on a single host; MAC addresses operate at the local network interface level; VLAN tags are about grouping frames into virtual LANs. So port numbers are the mechanism that enables multiple transport-layer conversations to coexist over one network path.