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Question:

This is the mechanism used to make sure that client application data being sent is received by the proper server application in a client-server network.

A port number
Explaination

To make sure the client application data is received by the right server application, port numbers are used. The CompTIA A+ exam would need you to understand several of the most commonly used and well-understand port numbers, such as Telnet, SSH, and SMTP.