Question
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?
Official answer and solution
- Official answer
- B
- From
- Official GATE 2024 answer key
Solution
Distinguish end-to-end Layer-3 IP addressing (destination host Y) from hop-by-hop Layer-2 MAC addressing (next-hop Layer-3 router R, since switch S is transparent at Layer 2).
- Destination IP address: Unless NAT is involved, the destination IP address in an IP datagram remains the end-to-end destination Y across all hops. Thus (B) is TRUE and (A) is FALSE.
- Destination MAC address: At Layer 2, node X addresses the Ethernet frame to the MAC address of the next-hop IP router R (not the Layer-2 switch S, which operates transparently without terminating the Ethernet frame, and not Y, which is in a different subnet behind R). Thus (C) and (D) are FALSE.
Option (B)
Thinking that an Ethernet switch S is addressed as the destination MAC address of frames traversing it.
The answer is the one in the official GATE answer key. The solution is this site's own explanation.
Source
- Paper
- GATE 2024 Computer Science and Information Technology - Set 2 (CS2)
- Question
- 23 of 65
- Section
- Computer Science and Information Technology
Report an error in this question
GATE questions are the work of the organising institutes. This site is independent and is not affiliated with them.