I've been reading Data Communication book by Forouzan it says that

"The physical addresses will change from hop to hop, but the logical addresses usually remain the same."

So, according to this line the physical address changes as the data passes through different devices. Now look at this figure:

fig: 2.20

Here the Router 1, changes its physical address from 20 to 99 while it was creating a new frame!

I think a router should have same physical address but how it changes? Please explain.

  • 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 Apr 1 '18 at 23:07

It's the physical address of the data packet which changes from hop to hop. the equipment is unchanged. The router in your example has two interfaces with two physical addresses.

I actually think it's clearer to think of the same layer 3 packet gets wrapped up in a new layer 2 frame for each hop. The new layer 2 frame has a new source and destination layer 2 address.

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(The following assumes a MAC-based link layer transport like Ethernet for simplicity. Other link layers may be used as well.)

Each physical address (aka MAC address) is only meaningful within its link layer segment (layer 2).

Packets using logical addresses (e.g. IP) need to be transported within L2 frames using physical addressing. For this the desired physical address has to be determined:

  • If the destination address is local (within L2 segment), the logical address is simply translated (ARP, NDP).
  • If the destination address is a non-local, remote address, a router is required and the physical address of the router is used to transport the packet to the router.

Essentially, a packet in flight is wrapped in a frame addressed to the next router (hop). Only in the final segment is the frame addressed to the destination node.

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