Final answer:
Each Telnet session between a Client and Server S is distinguished by unique source and destination port combinations, with clients using random high-numbered source ports and the server using Telnet port 23. So the correct answer is option D.
Step-by-step explanation:
When Client A initiates a Telnet session with Server S, and Client B does the same, different source and destination port numbers are used to distinguish between the sessions:
- a. The segments sent from A to S: Client A will use a random high-numbered source port (1024-65535), while Server S will use the well-known Telnet destination port 23.
- b. The segments sent from B to S: Similarly, Client B will use another random high-numbered source port, different from Client A's, while Server S’s destination port will again be 23.
- c. The segments sent from S to A: Server S will use source port 23, and the destination port will be the source port originally used by Client A.
- d. The segments sent from S to B: Server S will use source port 23, and the destination port will be the source port originally used by Client B.
This allows the server to maintain distinct Telnet sessions with each client, using the combination of source and destination ports to differentiate between the multiple connections.