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:
DEVICE IP ADDRESS MAC ADDRESS
WKS1 192.168.5.5 a-a-a-a-a-a
RTR1a 192.168.5.6 b-b-b-b-b-b
RTR1b 172.16.5.5 c-c-c-c-c-c
RTR2a 172.16.5.6 d-d-d-d-d-d
RTR2b 10.6.9.5 e-e-e-e-e-e
RTR3a 10.6.9.6 f-f-f-f-f-f
RTR3b 27.3.5.9 g-g-g-g-g-gWKS2 27.3.5.10 h-h-h-h-h-h
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.
Handoff Packet (IP) destination address Frame (MAC) Destination Address
WKS1 to RTR1a 27.3.5.10 b-b-b-b-b-b
RTR1b to RTR2a 27.3.5.10 d-d-d-d-d-d
RTR2b toRTR3a 27.3.5.10 f-f-f-f-f-f
RTR3b to WKS2 27.3.5.10 h-h-h-h-h-h
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 formathttps://serverfault.com/questions/438141/mac-address-changes-for-every-new-network
