Consider the 3 LANs interconnected by 2 routers as shown in Figure below. Assign

Question

Consider the 3 LANs interconnected by 2 routers as shown in Figure below. Assign IP addresses and MAC addresses for all the interfaces. Use address spaces 192.168.1.xxx for subnet 1, 192.168.2.xxx for subnet 2 and 192.168.3.xxx for subnet 3. Now consider the situation where host A is sending an IP datagram to host F. Give the source and destination MAC addresses (in the frame encapsulating this IP datagram) and the source and destination IP addresses of the IP datagram, as the frame is transmitted from node A to the left router, from left router to the right router and right router to node F. Now replace router between subnets 1 and 2 with a switch in figure 2 below. Label that switch S1 and the router between subnets 2 and 3 R1. Consider sending an IP datagram from host E to host F. Will host E ask router R1 to help forward the datagram? Why? In the Ethernet frame containing the IP datagram, what are the source and destination IP and MAC addresses? Suppose E would like to send an IP datagram to B, and assume that E's ARP cache does not contain B's MAC address. Will E perform an ARP query to find B's MAC address? Why? In the Ethernet frame (containing the IP datagram destined to B) that is delivered to router R1, what are the source and destination IP and MAC addresses? 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 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. What actions will switch S1 perform once it receives the ARP request message? Will router R1 also receive this ARP request message? If so, will R1 forward the message to subnet 3? Once host B receives this ARP request message, it will send back to host and ARP response message. But will it send an ARP query message to ask for A's MAC address? Why? What will switch S1 do once it receives an ARP response message from host B?

Explanation / Answer

a)

A- 192.168.1.001
     00-00-00-00-00-00

B- 192.168.1.003
     11-11-11-11-11-11

C- 192.168.2.001
     44-44-44-44-44-44

D- 192.168.2.004
     66-66-66-66-66-66

E- 192.168.3.001
     77-77-77-77-77-77

F- 192.168.3.003
    99-99-99-99-99-99

Interface 1 between Lan1 and router1
192.168.1.002
22-22-22-22-22-22

Interface 2 between Lan2 and router1
192.168.2.002
33-33-33-33-33-33

Interface 1 between Lan2 and router2
192.168.2.003
55-55-55-55-55-55

Interface 1 between Lan3 and router2
192.168.3.002
88-88-88-88-88-88

b)

Source IP:     192.168.1.001
Source MAC: 00-00-00-00-00-00

Destination IP:     192.168.3.003
Destination MAC: 99-99-99-99-99-99

c)
No because E can check the subnet prefix of Host F’s IP address, and learns that F is on the same LAN. Thus, E will not send the packet to the default router R1.
Ethernet frame from E to F:
Source IP = E’s IP address
Destination IP = F’s IP address
Source MAC = E’s MAC address
Destination MAC = F’s MAC address

d)
No, because they are not on the same LAN. E can find this out by checking B’s IP
address.
Ethernet frame from E to R1:
Source IP = E’s IP address
Destination IP = B’s IP address
Source MAC = E’s MAC address
Destination MAC = The MAC address of R1’s interface connecting to Subnet 3.

e)
i)Switch S1 will broadcast the Ethernet frame via both its interfaces as the received ARP frame’s destination address is a broadcast address. And it learns that A resides on Subnet
1 which is connected to S1 at the interface connecting to Subnet 1. And, S1 will update its forwarding table to include an entry for Host A.
ii)Yes, router R1 also receives this ARP request message, but R1 won’t forward the message to Subnet 3.
iii)B won’t send ARP query message asking for A’s MAC address, as this address can be obtained from A’s query message.
iv)Once switch S1 receives B’s response message, it will add an entry for host B in its forwarding table, and then drop the received frame as destination host A is on the same interface as host B (i.e., A and B are on the same LAN segment).

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.