Final answer:
In TCP/IP networking, a port number along with the IP address is used by the computer to identify which web browser should receive the datagram sent by a web server. Each browser instance will have a unique port number assignment that differentiates its traffic.
Step-by-step explanation:
The student is asking about TCP/IP protocol stack used in computer networking. When a user opens multiple web browsers and receives data from a web server, the TCP/IP protocol utilizes specific information to ensure that the response is directed to the correct browser. This information is Transport Layer information, which includes a combination of an IP address and a TCP or UDP port number. Every browser or tab communicating through the network will have a unique port number assigned to its session, which distinguishes it from others. This port number is used by the underlying operating system and network card to direct the incoming data to the correct process or web browser instance. Therefore, when the Cisco server sends a datagram, this port number along with the IP address helps the PC's network stack route the reply correctly to the browser that initiated the request.