When a packet is forwarded through a network from one host to another host, which of the following fields in the Ethernet frame will change at every hop?

A. Source IP address

B. Destination MAC address

C. Source port number
D. Destination IP address

Explanation:

When an Ethernet frame is forwarded through the network, both the source and destination MAC addresses will change at every hop.

The source and destination IP addresses and source and destination port numbers MUST remain the same for proper routing to occur, for the proper delivery to the destination service, and for the proper reception of responses to the sending device. By contrast, the MAC addresses used at each hop must be those of the physical interfaces involved in the Layer 2 forwarding at each hop.

As a simple illustration of this process, IP addresses and MAC addresses are assigned to two computers and three routers shown in the diagram. The network is arranged as shown below:

The IP addresses and the MAC addresses of each device are shown below:

There will be four handoffs to get this packet from WKS1 to WKS2. The following table shows the destination IP addresses and destination MAC addresses used at each handoff.

As you can see, the destination IP address in the packet does not change, but the MAC address in the frame changes at each handoff.

Objective:
LAN Switching Fundamentals
Sub-Objective:
Interpret Ethernet frame format

References:
MAC address changes for every new network