What is the role of port numbers in the transport layer?

• A. Ephemeral ports identify standard services on a host.
• B. Port numbers identify specific processes or services on a host; well-known ports map to standard services; ephemeral ports for clients.
• C. Port numbers indicate the packet's encryption.
• D. Port numbers identify the physical network interface.

Answer: (B)

Port numbers in the transport layer identify which process or service on a host should handle a given segment. A host runs many processes that communicate over the network, and each service binds to a specific port number. Well-known ports (the first range, typically 0–1023) are reserved for standard services, so clients know which port to contact for a given service—for example, a web server commonly uses port 80. When a client starts a connection, it uses an ephemeral (temporary) source port, which lets the host distinguish this particular conversation and ensures replies reach the correct client socket. On the receiving end, the destination port tells the server-side process or service to handle the request, while the source port helps the client match the response to the original request. This design enables numerous simultaneous conversations between hosts, multiplexed over the same IP addresses.

Port numbers do not indicate encryption, nor do they identify a physical network interface.