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#
Authors:
James F. Kurose, Keith W. Ross
Chapter:
The Link Layer: Links,access Networks, And Lans
Exercise:
Problems
Question:14 | ISBN:9780132856201 | Edition: 6

Question

Consider three LANs interconnected by two routers, as shown in Figure 5.33. a. Assign IP addresses to all of the interfaces. For Subnet 1 use addresses ofthe form 192.168.1.xxx; for Subnet 2 uses addresses of the form 192.168.2.xxx; and for Subnet 3 use addresses of the form 192.168.3.xxx.

b. Assign MAC addresses to all of the adapters.

c. Consider sending an IP datagram from Host E to Host B. Suppose all of the ARP tables are up to date. Enumerate all the steps, as done for the single- outer example in Section 5.4.1.
d. Repeat (c), now assuming that the ARP table in the sending host is empty

(and the other tables are up to date).

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Answer

a)

To assign IP addresses to the interfaces in the three LANs, following the given format:

Subnet 1:

  • Router 1 Interface 0: 192.168.1.1
  • Host A: 192.168.1.2
  • Host B: 192.168.1.3

Subnet 2:

  • Router 1 Interface 1: 192.168.2.1
  • Router 2 Interface 0: 192.168.2.2
  • Host C: 192.168.2.3
  • Host D: 192.168.2.4

Subnet 3:

  • Router 2 Interface 1: 192.168.3.1
  • Host E: 192.168.3.2
  • Host F: 192.168.3.3

 

b)

Assigning MAC addresses to the adapters:

  • Router 1 Interface 0: MAC_A
  • Router 1 Interface 1: MAC_B
  • Router 2 Interface 0: MAC_C
  • Router 2 Interface 1: MAC_D
  • Host A: MAC_E
  • Host B: MAC_F
  • Host C: MAC_G
  • Host D: MAC_H
  • Host E: MAC_I
  • Host F: MAC_J

 

c)

Steps for sending an IP datagram from Host E to Host B:

  1. Host E checks its ARP table to find the MAC address for 192.168.3.1 (Router 2 Interface 1). Suppose the MAC address is MAC_D.
  2. Host E encapsulates the IP datagram destined for Host B inside an Ethernet frame.
    • Source IP: 192.168.3.2 (Host E's IP address)
    • Destination IP: 192.168.1.3 (Host B's IP address)
    • Source MAC: MAC_I (Host E's MAC address)
    • Destination MAC: MAC_D (Router 2 Interface 1's MAC address)
  3. Host E sends the Ethernet frame onto its LAN.
  4. The Ethernet frame reaches Router 2 Interface 1.
  5. Router 2 checks its ARP table to find the MAC address for 192.168.1.3 (Host B). Suppose the MAC address is MAC_F.
  6. Router 2 encapsulates the received Ethernet frame inside a new Ethernet frame.
    • Source IP: 192.168.3.1 (Router 2 Interface 1's IP address)
    • Destination IP: 192.168.1.3 (Host B's IP address)
    • Source MAC: MAC_D (Router 2 Interface 1's MAC address)
    • Destination MAC: MAC_F (Host B's MAC address)
  7. Router 2 sends the new Ethernet frame onto its LAN.
  8. The Ethernet frame reaches Host B.
  9. Host B receives the frame, extracts the IP datagram, and processes it.

 

d)

Assuming the ARP table in the sending host (Host E) is empty:

  1. Host E checks its empty ARP table and realizes it doesn't have the MAC address for 192.168.3.1 (Router 2 Interface 1).
  2. Host E sends an ARP broadcast message:
    • Source IP: 192.168.3.2 (Host E's IP address)
    • Source MAC: MAC_E (Host E's MAC address)
    • Target IP: 192.168.3.1 (Router 2 Interface 1's IP address)
  3. The ARP broadcast reaches Router 2 Interface 1, which learns Host E's MAC address (MAC_E) from the ARP message.
  4. Router 2 Interface 1 sends an ARP unicast reply back to Host E:
    • Source IP: 192.168.3.1 (Router 2 Interface 1's IP address)
    • Source MAC: MAC_D (Router 2 Interface 1's MAC address)
    • Target IP: 192.168.3.2 (Host E's IP address)
    • Target MAC: MAC_E (Host E's MAC address)
  5. Host E receives the ARP reply, updates its ARP table, and now knows the MAC address of Router 2 Interface 1.
  6. Host E encapsulates the IP datagram destined for Host B inside an Ethernet frame.
    • Source IP: 192.168.3.2 (Host E's IP address)
    • Destination IP: 192.168.1.3 (Host B's IP address)
    • Source MAC: MAC_E (Host E's MAC address)
    • Destination MAC: MAC_D (Router 2 Interface 1's MAC address)
  7. Host E sends the Ethernet frame onto its LAN.
  8. The Ethernet frame follows the same steps as in part c from Step 4 onwards to reach Host B.
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