What is the distinction between well-known and ephemeral port numbers and why are they used?

• A. Ephemeral ports are used to identify standard services on a host.
• B. Well-known ports (0–1023) are assigned to dynamic, ephemeral services.
• C. Ephemeral ports are permanently assigned to servers.
• D. Well-known ports (0–1023) map to standard services; ephemeral ports are temporary client-side ports for individual connections.

Answer: (D)

The main idea is that ports serve different roles for services vs. clients. Well-known ports (0–1023) are fixed to standard services on servers; a server listens on a specific well-known port so clients know where to connect for that service (for example, HTTP on port 80, SSH on 22). Ephemeral ports are temporary, high-numbered ports chosen by a client to identify its end of a specific connection. When a client opens a connection to a server, it uses a temporary source port from the ephemeral range, while the server continues to listen on the well-known destination port. This pairing—source ephemeral port, destination well-known port—along with IP addresses, forms a unique connection identifier, allowing many simultaneous connections to the same server service without port conflicts. Ephemeral ports are not tied to a particular service and are released after the session ends, often becoming available again after a short delay. This arrangement enables a single server to handle many clients concurrently and supports multiple concurrent connections from one client by using different ephemeral source ports.