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I'm a newbie to networking. I wanted to ask this:

When a transmitting device (host A) wishes to send packets to a host (B) which is on a remote network (LAN), (I understand that the datagrams are encapsulated by each layer while going down), I know routers are only concerned with IP addresses from the network layer, but what I don't understand is that the Cisco CCNA book says that when the packet reaches the receiving host's (B) router, the frame's control information is stripped off, and the packet is framed again for sending to host B. If this is true, what is in the control information in a frame? What is changed? Does the source hardware address from Host A change? If so, How? What source address is it replaced by?

I may sound confusing but I don't get the gist of this whole stripping of frames subject.

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When a layer-2 frame reaches a layer-3 device, like a router, the layer-2 frame is stripped off so that the router can read the layer-3 information. When the router switches the packet to a new interface, it creates a new frame for the new link. The information in the old frame headers (MAC addresses, etc.) were lost when the frame was stripped off, so the router creates a new frame with its own MAC address as the source address, and the other end of the link's MAC address as the destination address.

This all assumes that the intermediate links are something like ethernet, with MAC addresses. Other layer-2 protocols may be in a link, and not all layer-2 protocols use MAC addresses.

  • Hey, thanks for the brilliant answer, but I can't favourite it for another 4 minutes lol. Is the Source IP (Host A's router) stripped off and replaced with Host B's IP address too? Or is that kept intact? – Akira May 12 '16 at 19:17
  • No, IP addresses are not changed from end-to-end. This is broken if NAT (Network Address Translation) is used. – Ron Maupin May 12 '16 at 19:24

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