A client packet is received by a server. The packet has a destination port number of 143. What service is the client requesting?

The client is requesting the IMAP (Internet Message Access Protocol) service. In TCP/IP networking, port numbers are used to distinguish different services. Port number 143 is specifically assigned to the IMAP service, which is used for retrieving emails from a mail server.

This is different from the other services you mentioned:

• Telnet, typically used for remote command-line access, uses port 23.
• SSH (Secure Shell), used for secure remote access, uses port 22.
• FTP (File Transfer Protocol), used for transferring files, uses ports 20 and 21.