Which of the following are examples of the client–server model in computer networks, where a client program requests a service and a server program responds?

Introduction / Context:
The client–server model underpins most Internet applications. A client initiates a request, and a server listens and responds. Understanding which common protocols follow this pattern builds intuition about how applications interact over TCP/IP networks.

Given Data / Assumptions:

• Protocols considered: TELNET, FTP, and DNS.
• Definition: client requests, server provides the requested service.

Concept / Approach:
TELNET clients connect to a TELNET server for remote terminal sessions. FTP clients connect to FTP servers for file operations. DNS resolvers (clients) query authoritative or recursive DNS servers to translate names to addresses. Each follows the core client–server pattern of request/response.

Step-by-Step Solution:

Verification / Alternative check:
Port/service mappings show well-known server ports (e.g., 21 for FTP control, 23 for TELNET, 53 for DNS) with client ephemeral ports making requests, confirming client–server behavior.

Why Other Options Are Wrong:

• Any single option alone is incomplete; all three are valid examples.
• None of the above: invalid since each listed protocol uses client–server communication.

Common Pitfalls:
Confusing peer-to-peer with client–server; mixing DNS client caching with authoritative service—both still follow a request/response model.

Final Answer:
All of the above