GATE CS 2024 Set 2 Q13 — Data Link Layer: Framing, Error Detection, MAC, Ethernet Bridging

To determine the correct destination IP and MAC addresses when the packet leaves Node $\text{X}$, we analyze the roles of the Network Layer (Layer 3) and the Data Link Layer (Layer 2):

1. Destination IP Address (End-to-End):

   IP addressing is responsible for end-to-end delivery. The source IP is $\text{X}$ and the destination IP is the final destination, which is $\text{Y}$. This IP address remains unchanged as the packet travels across intermediate routers. Thus, the destination IP address is the IP address of $\text{Y}$.

2. Destination MAC Address (Hop-by-Hop):

   MAC addressing is responsible for hop-by-hop delivery within a local network segment.

   • Since $\text{Y}$ is on a different network (connected via router $\text{R}$), $\text{X}$ cannot send the frame directly to $\text{Y}$'s MAC address. Instead, $\text{X}$ must send it to its default gateway, which is the router $\text{R}$. Therefore, the destination MAC address is the MAC address of $\text{R}$.
   • The Ethernet switch $\text{S}$ is a transparent Layer 2 device. It forwards frames based on their destination MAC address but is not the destination itself. Hence, the destination MAC address is not that of $\text{S}$.

Comparing this with the given options:

• A is False: The destination IP is of the final host $\text{Y}$, not the intermediate router $\text{R}$.
• B is True: The destination IP is indeed the IP address of $\text{Y}$.
• C is False: The destination MAC is that of the next-hop router $\text{R}$, not the transparent switch $\text{S}$.
• D is False: The destination MAC is that of the router $\text{R}$, not the final host $\text{Y}$ (which is on a different network).