Consider the network shown in the figure below and suppose that initially all of

Question

Consider the network shown in the figure below and suppose that initially all of the ARP tables are up to date at each host. Answer the following questions:

a) How many subnets are there? List those subnets in the form a.b.c.d/x where x is the subnet mask.

b) Consider sending an IP datagram from Host E to Host F.

1. Will Host E ask router R1 to help forward the datagram? Why?

2. In the Ethernet frame containing the IP datagram, what are the source and destination IP and MAC addresses?

c) Now suppose Host E would like to send an IP datagram to B, and assume that E's ARP cache does not contain B's MAC address.

1. Will E perform an ARP query to find B's MAC address? Why?

2. In the Ethernet frame containing the IP datagram destined to B, what are the source and destination IP and MAC addresses?

d) Now suppose Host A would like to send an IP datagram to Host B, and neither A's ARP cache contains B's MAC address nor does B's ARP cache contain A's MAC address. Further suppose that the switch S1's forwarding table contains entries for Host B and router R1 only. Thus, A will broadcast an ARP request message.

1. What will switch S1 do with the ARP request message once it receives it? Will S1 update its forwarding table? Is so, how?

2. Will R1 also receive this ARP request message? If, so will R1 forward the message through Interface 1?

3. Once Host B receives this ARP request message, it will send back to Host A an ARP response message. But will it send an ARP query message to ask for A's MAC address? Why?

4. What will switch S1 do with the ARP response message from Host B once it receives it? Will switch S1 update its forwarding table? If so, how?

Host C IP: 192.168.1.4 MAC: 33-33-33-33-33-33 Host A IP: 192.168.1.2 MAC: 11-11-11-11-11-11 Host E IP: 192.168.2.2 MAC: 55-55-55-55-55-55 R1 Interface 1 IP: 192.168.2.1 R1 MAC: 88-88- 88-88-88-88 R1 Interface 0 IP: 192.168.1.1 MAC: 77-77- 77-77-77-77 S1 Host B IP: 192.168.1.3 MAC: 22-22-22-22-22-22 Host F IP: 192.168.2.3 MAC: 66-66-66-66-66-66 Host D IP: 192.168.1.5 MAC: 44-44-44-44-44-44

Explanation / Answer

(B) Consider sending an IP datagram from Host E to Host F.
1) Will Host E ask router R1 to help forward the datagram? Why?
Ans- The answer for this question is No. Because both the host are belonging to the same network and therefore there will be no involvement of the Router R1 in this case.
It is one of the case where the services of the ARP is used.
      
Note:- For this type of the cases, i.e if the sender is host and want to send the packet to another host on the same network then, on that case, the logical address that must be mapped
to the physical address is the destination IP address in the datagram number.

2)In the Ethernet frame containing the IP datagram, what are the source and destination IP and MAC addresses?
Ans- As mentioned in the point earlier, the frame containing the IP datagram will have the logical address of both source and the destination system.However, initially when the source wants to send the packet to destination, the MAC address of the destination is sent along, the intended recipient identifies the IP and send the packet back to the source including its own IP address.

Thus, the Source IP- 192.168.2.2 , Destination IP- 192.168.2.3

    SOurce MAC address - 55-55-55-55-55-55,  Destination MAC address- 66-66-66-66-66-66
      
C) Now suppose Host E would like to send an IP datagram to B, and assume that E's ARP cache does not contain B's MAC address.
1) Will E perform an ARP query to find B's MAC address? Why?
Ans- Each time the sender wants to send the data to the destination, the check is always done for the destination and then only the source is able to send the data to the destination, only after receiving the IP address of the recipient.
   The concept of the cache was introduced only to save the time for this check which was made by the sender every time..
   Now if the E's ARP cache do not have the B's MAC address, then the source E will have to enquire again about the B's MAC address, to send the data to the destination.
   As the sender do not have any info on the cache about the data transfer, it will enquire again to fid the destination.
     
2) In the Ethernet frame containing the IP datagram destined to B, what are the source and destination IP and MAC addresses?
   Ans- SOurce IP- 192.168.2.2
       Destination IP- 192.168.1.3
         
       Source MAC- 55-55-55-55-55-55
       Destination MAC- 22-22-22-22-22-22
         
D) Now suppose Host A would like to send an IP datagram to Host B, and neither A's ARP cache contains B's MAC address nor does B's ARP cache contain A's MAC address.
Further suppose that the switch S1's forwarding table contains entries for Host B and router R1 only. Thus, A will broadcast an ARP request message.
   1) What will switch S1 do with the ARP request message once it receives it? Will S1 update its forwarding table? Is so, how?
   Ans- As we can see in the network diagram that the Host A and host B are on the same network.Thus, the packet enquired by the source will be sent to all the destined recipient.
   Now as the switch S1 already have the entries for the Host B, the table will not be updated in this case because it already have the information about the destiantion.
       Whereas in return the the data will be fetched from the switch itself and datagram will be send to the receiver.
         
   2) Will R1 also receive this ARP request message? If, so will R1 forward the message through Interface 1?
   Ans- Generally as we talk about the forwarding of the messages, the request will be sent to all the systems, no matter whether if is through the router
   or by the switches.
       But, it is just the overhead on the network. Here in this case the information have been received from the switch itself but the packet will be forwarded to Router R1
       but it will be simply ignored by the system.
         
   3)Once Host B receives this ARP request message, it will send back to Host A an ARP response message. But will it send an ARP query message to ask for A's MAC address? Why?
   Ans- If it would have send the data packet through the router, then the Host B would have asked for the MAc address of the source. As this packet is sent within the network,
   the destined receiver will not ask for the MAC address of the sender.
         
   4)What will switch S1 do with the ARP response message from Host B once it receives it? Will switch S1 update its forwarding table? If so, how?
   Ans- AS the switch S1 already have the information about the Host B in the table, it will update the forwarding table with A's entry in it because the packet is being sent
   to the Host A now, which in case needs the table updation.

A) How many subnets are there? List those subnets in the form a.b.c.d/x where x is the subnet mask.
Ans- There are altogether 2 subnets, that is there are two networks. Also we can see from the network diagram that both the networks are connected by router and also both have different
prefixes.
   The subnet mask is /24.
   The subnets are- 192.168.1.1/24 and 192.168.2.1/24

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