Node X has a TCP connection open to node Y. The packets from X to Y go through…
2024
Node X has a TCP connection open to node Y. The packets from X to Y go through an intermediate IP router R. Ethernet switch S is the first switch on the network path between X and R. Consider a packet sent from X to Y over this connection.
Which of the following statements is/are TRUE about the destination IP and MAC addresses on this packet at the time it leaves X?
Answer: B. The destination IP address is the IP address of Y — Answer: The destination IP is the IP address of Y. Key points: IP destination is end-to-end: the sender sets the IP destination to Y and routers forward…
- A.
The destination IP address is the IP address of R
- B.
The destination IP address is the IP address of Y
- C.
The destination MAC address is the MAC address of S
- D.
The destination MAC address is the MAC address of Y
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Correct answer: B
Answer: The destination IP is the IP address of Y.
Key points:
IP destination is end-to-end: the sender sets the IP destination to Y and routers forward packets toward that IP without changing it.
Ethernet (MAC) destination is local to the current link: it is the next-hop's MAC address. If Y is on the same local network, that MAC is Y's. If Y is on a different network, the MAC is the router R's MAC (the next-hop gateway).
When leaving X in the common multi-hop case (X -> switch S -> router R -> ... -> Y), the packet will have IP destination = Y and Ethernet destination = R (the next-hop gateway); ARP is used to learn the appropriate MAC.
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