1. This question is based on the figure shown below. The figure shows the topolo
ID: 3708459 • Letter: 1
Question
1. This question is based on the figure shown below. The figure shows the topology of the network, and the assigned IP address and MAC address.
a. Consider sending an IP datagram from Host E to Host D. Will Host E ask router R1 to help forward the datagram? In the Ethernet frame containing the IP datagram, what are the source and destination IP and MAC addresses?
b. In the Ethernet frame (containing the IP datagram destined to F), what are the source and destination IP and MAC addresses?
c. Suppose Host E would like to send an IP datagram to Host F, and neither E’s ARP cache contains F’s MAC address nor does F’s ARP cache contain entries for Host F and router R1 only. Thus, E will broadcast an ARP response message. But will it send an ARP query message to ask for E’s MAC address? Why? In the Ethernet frame (containing the IP datagram destined to F), what are the source and destination IP and MAC addresses?
A192.168.1.001 ? 19H68·2.001 192.168.3.001 77-77-77-77-77-77 00-00-00-00-00-00 44-44-44-44-44-44 LAN Router 1 LAN Switch 1 LAN 92.168.1.602 22-22-22-22-22-22 33-33-33-33-33-33 92.168.2.002 192.168.3.005 88-88-88-88-88-88 192.168.2.004 B192.168.1.003 D 66-66-66-66-66 192.168.3.003 F 99-99-99-99-99-99Explanation / Answer
a. Consider sending an IP datagram from Host E to Host D. Will Host E ask router R1 to help forward the datagram? In the Ethernet frame containing the IP datagram, what are the source and destination IP and MAC addresses?
Answer:
No. Between host D and host E, switch is used for interconnection. So, both the hosts belong to same LAN.
E can check the subnet prefix of Host D’s IP address, and then learn that host E is on the same LAN. Thus, E will not send the packet to the router R1.
Ethernet frame from E to D:
Source IP = 192.168.3.001
Destination IP = 192.168.2.004
Source MAC = 77-77-77-77-77-77
Destination MAC = 66-66-66-66-66-66
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b. In the Ethernet frame (containing the IP datagram destined to F), what are the source and destination IP and MAC addresses?
Answer: The question seems to be incomplete. Which is the source host of the IP datagram
Let's Assume Source node is E and IP datagram is destined to F.
Then, Ethernet frame from E to F:
Source IP = 192.168.3.001
Destination IP = 192.168.3.003
Source MAC = 77-77-77-77-77-77
Destination MAC = 99-99-99-99-99-99
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c. Suppose Host E would like to send an IP datagram to Host F, and neither E’s ARP cache contains F’s MAC address nor does F’s ARP cache contain entries for Host F and router R1 only. Thus, E will broadcast an ARP response message. But will it send an ARP query message to ask for E’s MAC address? Why? In the Ethernet frame (containing the IP datagram destined to F), what are the source and destination IP and MAC addresses?
Answer:
Switch S1 will broadcast the Ethernet frame on the interface to subnet 1 (since it got the frame on its interface to subnet 2) as the received ARP frame’s destination address is a broadcast address. And it learns that E resides on Subnet 2 which is connected to S1 at the interface connecting to Subnet 2. And, S2 will update its forwarding table to include an entry for Host E.
Router R1 also receives this ARP request message, but R1 won’t forward the message to other Subnet. F won’t send ARP query message asking for E’s MAC address, as this address can be obtained from E’s query message. Once switch S1 receives F’s response message, it will not add an entry for host F to its forwarding table (as the question says explicitly that S1 already had an entry for host F), and then drop the received frame as destination host E is on the same interface as host F (i.e., E and F are on the same LAN segment).
Ethernet frame from E to F:
Source IP = 192.168.3.001
Destination IP = 192.168.3.003
Source MAC = 77-77-77-77-77-77
Destination MAC = 99-99-99-99-99-99
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d. Suppose now that the router 1 is replaced by a switch. Suppose A would like to send an IP datagram to D, and assume that A’s ARP cache does contain D’s MAC address. In the Ethernet frame, what are the source and destination IP and MAC addresses?
Answer:
Host A doesn't have D's Mac address in its ARP cache, so it will broadcast the ARP request to the switch. The switch will broadcast the ARP query and Host D when it recieves the ARP query, Its sends back the ARP response back to Switch and finally Host A will learn its path to reach D.
Ethernet frame from A to D:
Source IP = 192.168.1.001
Destination IP = 192.168.2.004
Source MAC = 00-00-00-00-00-00
Destination MAC = 66-66-66-66-66-66
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