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)
Explanation
IP addresses name the two ends of the journey, while a MAC address only names the next stop on the current link.
- The IP header carries the final destination, so the destination IP address is Y's all the way; routers forward on it but do not rewrite it. B is true and A is false.
- Y is on another network, so X sends the frame to its next-hop router R: the destination MAC address is R's, which X finds through ARP. R then puts the packet in a new frame for the next link.
- An Ethernet switch is transparent. It forwards frames by their destination MAC address but is never itself the addressee, so C is false. Y's MAC address would be used only if Y were on X's own LAN, so D is false.
Answer: B