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(warning: this is an old exam question, for which I don't have a solution):

Assume 2 local networks LAN1 and LAN2, with Endsystems A,C in LAN1 and B,D in LAN2. The networks are coupled with a bridge. Assume A wants to send a TCP Frame to B. Which address information does it need to contain? List all headers of OSI Layers 2-4, take sender and receiver into account.

What my solution would be:

TCP Port A -> IP-A -> Mac-A -> Mac-Bridge -> Mac-B -> IP-B -> TCP-Port-B is that correct?

If I used a Router instead of the bridges and LAN1 and LAN2 are seperate IP-Subnetworks, would the difference be:

TCP Port A -> IP-A -> Mac-A -> Mac-Bridge -> IP-Bidge-> Mac-Bridge -> Mac-B -> IP-B -> TCP-Port-B?

Or what would change here? Am I understanding this correctly?

Thank you!

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  • Is this a homework question? What's the application you're building or problem you're solving? Commented Aug 4, 2015 at 13:45
  • Like I said on top: I'm preparing for an exam, this is an old exam question for which there is no official solution. I would like to verify my understanding of the subject. Commented Aug 4, 2015 at 14:20

1 Answer 1

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The packet will contain

Case 1 - No routing - PC A send packet directly to PC B

  • source TCP port
  • destination TCP port
  • source IP address : IP address of A
  • destination IP address : IP address of B
  • source MAC addres : MAC address of A
  • destination MAC address: MAC address of B (obtained trough an ARP request on B IP address)

Case 2 - With router - the PC A send the packet to its gateway (I.E. the router)

  • source TCP port
  • destination TCP port
  • source IP address : IP address of A
  • destination IP address : IP address of B
  • source MAC addres : MAC address of A
  • destination MAC address: MAC address of the router (obtained trough an ARP request on the router IP address. I.E. the IP configured as defaut gateway on A)
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  • Assuming LAN1 and LAN2 are different subnets, with no router to allow traffic between subnets, then in Case 1 PCa won't send the traffic to PCb. It will want to send the traffic to a gateway or if no gateway is configured give a "no route to host" result. Commented Aug 12, 2015 at 2:34

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