Trace the Packets

This week the CCNA Virtual Study Group focused on Packet Tracing. Let’s break down various questions about which Source/Destination MAC & IP addresses get used when a host sends a packet across the network.

Question One

PC1 is on your organization’s network. Router A is a NAT router. PC1 sends a packet to PC2, and PC2 sends a packet back to PC1.

Which is an outside local address?

a. 20.28.40.109

b. 192.168.70.33

c. 30.21.75.55

d. 192.168.140.74

The Breakdown

Key Points:

  • PC1 is on YOUR organization’s network (inside the network)
  • PC2 is out on the internet somewhere (outside the network)
  • RouterA is a NAT route

What We Know:

  • RFC 1918 tells us that 192.168.70.33 and 192.168.140.74 are private IP addresses (not to be publicly routed on the internet)
  • PC1 is sending a packet to PC2
  • PC1 is inside the network

Process of Elimination:

  • 20.28.40.109 and 30.21.75.55 are public IP Addresses and we are looking for LOCAL inside IP Addresses
  • Answers A & C can be eliminated
  • 192.168.70.33 is PC1, making it a LOCAL INSIDE address
  • 192.168.140.74 is PC2, which is outside the network, and therefore the correct answer

Question Two

Using the data output below. For a packet leaving the FastEthernet 0/0 interface of R2:

Which of the following will be used as the source IP address?

a. 192.168.172.74

b. 192.168.172.1

c. 192.168.172.33

d. 192.168.172.2

 

Which of the following will be the MAC address of the packet as it exits FastEthernet 0/1 interface of R1?

a. 431d.efd6.fc43

b. 5849.3b60.4b12

c. 0100.5e7f.fffa

d. 9353.53c3.ff02

 

Data Output from Devices

PC1
C:>ipconfig
<omitted output>
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 192.168.172.33
Subnet Mask . . . . . . . . . . . : 255.255.255.240
Default Gateway . . . . . . . . . : 192.168.172.1


PC2
C:>ipconfig
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 192.168.172.74
Subnet Mask . . . . . . . . . . . : 255.255.255.248
Default Gateway . . . . . . . . . : 192.168.172.2


R1
RouterA#show ip arp
Protocol Address Age (min) Hardware Addr Type
Interface Internet 192.168.172.1 - 5849.3b60.4b12 ARPA FastEthernet0/0
Internet 192.168.172.17 - 0100.5e7f.fffa ARPA FastEthernet0/1


R2
RouterB#show ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 192.168.172.2 - c85b.762c.f7cb ARPA FastEthernet0/0
Internet 192.168.100.18 - 0100.e3eb.60f4 ARPA FastEthernet0/1

The Breakdown

(question one) Key Points:

  • Only 2 hosts are present in the diagram
  • The packet is exiting R2 fa 0/0 interface

(question one) What We Know:

  • 192.168.172.2 is the IP Address for R2 fa 0/0 interface
  • 192.168.172.2 is the Default Gateway for PC2
  • The packet is moving from the left of the diagram (PC1)  to the right of the diagram (PC2)

(question one) Process of Elimination:

  • 431d.efd6.fc43 and 9353.53c3.ff02 belong to the HOSTS looking at the diagram and can be eliminated
  • 5849.3b60.4b12 is the MAC Address for R1 fa 0/0 (we are looking for R1 fa 0/1)
  • 0100.5e7f.fffa would be the best answer since it is the MAC address for R1 fa 0/1