I'm confused with how NAT and routers work in ethernet standards. In the image below, when a request is sent from Client to Server, by the time the request packets arrive R2, then what are source IPv4 address, source MAC address, destination IPv4 address, and destination MAC address of the packets. My own answer is:

  • source IP: inside global IP address of R1

  • source MAC: Client's MAC address

  • des IP: Server's IP address

  • des MAC: Server's MAC address

Am I right? If not, correct me.R1 is a NAT server

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 14, 2017 at 13:52

1 Answer 1


This question is really off-topic here. All "education, certification, or homework" questions are explicitly off-topic here. Your answer is partially correct. I'm not going to do your work for you, but I will explain how to arrive at the correct answer.

Don't confuse different network layers. Ethernet is a layer-1/2 set of protocols, while NAT works at layer-3/4. Packets are layer-3 datagrams, and they contain the source and destination layer-3 (e.g. IPv4) addresses. Frames are layer-2 datagrams, and they contain the source and destination layer-2 (e.g. MAC) addresses. Packets are encapsulated by frames before being placed on the wire.

Ethernet doesn't care what upper-layer protocols it carries. It can carry various (IPv4, IPX, IPv6, etc.) layer-3 protocols, and it doesn't care. The upper-layer protocols are just the payload of ethernet frames.

Also, IPv4 doesn't care which layer-1/2 protocol carries it. IPv4 can be encapsulated by many different layer-2 protocols. In your diagram, the link between the two routers could be something like frame-relay over a serial connection, and that doesn't use MAC addresses at all.

MAC addresses are confined to the LAN on which they originate: they are the addresses used on layer-2 domain, but they do not cross layer-3 devices. When a layer-2 frame gets to a layer-3 device, e.g. a router, the frame is stripped, the packet is switched to the next interface, and a new frame for the new interface is created to encapsulate the packet. The layer-3 address is translated before the new frame encapsulation happens.

  • This seems like a very valid question. I don't mean to disrespect, but this forum should be more open. I think there is a reason that networkengineeringSE lags in participation. I can see someone finding the answer/explanation to this question valuable. Why not leave it? It's a good question, in my opinion. Again, no disrespect, ...just my 2 cents. Jun 5, 2016 at 22:32
  • @RonRoyston, what is it you are saying? I answered the question, and it currently has no close votes. I'm not going to do someone else's homework, so I explained enough for the OP to come up with the answer.
    – Ron Maupin
    Jun 5, 2016 at 22:35
  • I'm saying you told the guy his question is really it's off topic and that you should not have. That's all. It's just my opinion. Maybe you're right. I just think it's a good question. Jun 5, 2016 at 22:38
  • I believe I explained that it is off topic because it is, and that is the reason I wouldn't directly answer the question. I have no problem explaining the theory, but we don't answer those types of questions. Often textbooks have typos, and instructors are teaching something that doesn't directly relate to the real world. If we give an answer, even a correct one, it may be wrong for the class. Remember, "Network Engineering Stack Exchange is for asking questions about professionally managed networks in a business environment."
    – Ron Maupin
    Jun 5, 2016 at 22:43

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