1. 5.2.1.4 - MAC Address Tables on Connected Switches

2. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0B
Source MAC
00-0A
Type
Data
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
PC-A sends a frame to PC-B
The fame is received by switch S1

3. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet 1
00-0A S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0B
Source MAC
00-0A
Type
Data
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
Switch S1 sees if it has something to learn by examining the source MAC address
The source MAC address is not in its MAC address table
S1 adds the source MAC address 00-0A and the incoming port 1 to its MAC address table

4. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet 1
00-0A S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0B
Source MAC
00-0A
Type
Data
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
S1 can now forward the frame by examining the destination MAC address
The destination MAC address 00-0B is not in its MAC address table, so it floods it out all ports
This is known as an unknown unicast

5. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet 1
00-0A S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0B
Source MAC
00-0A
Type
Data
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
PC-B’s NIC receives the frame.
After comparing the destination MAC address of 00-0B to it own NIC’s MAC address it realizes they are the same and copies in the rest of the frame.

6. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet 1
00-0A 1
00-0A S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0B
Source MAC
00-0A
Type
Data
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
The frame is also flooded out S1’s port 4 which is connected to switch S2
S2 receives the frame and examines the source MAC address
Since this is new information, S2 add the source MAC address 00-0A and the incoming port number 1 to its MAC address table

7. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet 1
00-0A 1
00-0A S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0B
Source MAC
00-0A
Type
Data
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
S2 can now forward the frame by examining the destination MAC address
The destination MAC address 00-0B is not in its MAC address table, so it floods it out all ports
Again, this is known as an unknown unicast

8. X X S1 MAC Address Table S2 MAC Address Table Port MAC Address Port
Internet 1
00-0A 1
00-0A S1 1 2 3 4 S2 1 2 3 4
Router 1 2 X
MAC
00-0D A B C X
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0B
Source MAC
00-0A
Type
Data
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
Both PC-C’s NIC and the Router’s NIC receives the frame.
Since their MAC addresses do not match the destination MAC address they drop the rest of the frame

9. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet 1
00-0A 1
00-0A S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0A
Source MAC
00-0B
Type
Data
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
PC-B now sends the frame to PC-A
The frame is received by switch S1

10. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet 1
00-0A 1
00-0A 3
00-0B S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0A
Source MAC
00-0B
Type
Data
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
Switch S1 sees if it has something to learn by examining the source MAC address
The source MAC address is not in its MAC address table
S1 adds the source MAC address 00-0B and the incoming port 3 to its MAC address table

11. S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address
Internet 1
00-0A 1
00-0A 3
00-0B S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0A
Source MAC
00-0B
Type
Data
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
S1 can now forward the frame by examining the destination MAC address
The destination MAC address 00-0A is in its MAC address table, so it filters the frame by sending it out only port 1

12. 5.2.1.5 - Sending a Frame to the Default Gateway

13. Destination IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0D
Source MAC
00-0A
Type
Data
Destination IP address on a remote network
FCS
Destination IP address is on a remote network.
PC-A will send the Ethernet frame to the MAC address of the default gateway

14. Destination IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0D
Source MAC
00-0A
Type
Data
Destination IP address on a remote network
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
Switch S1 sees if it has something to learn by examining the source MAC address
The source MAC address is in its MAC address table, so it has nothing new to learn
S1 refreshes the 5 minute refresh timer for this entry

15. Destination IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0D
Source MAC
00-0A
Type
Data
Destination IP address on a remote network
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
S1 can now forward the frame by examining the destination MAC address
The destination MAC address 00-0D is not in its MAC address table, so it floods it out all ports
This is known as an unknown unicast

16. Destination IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D X A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0D
Source MAC
00-0A
Type
Data
Destination IP address on a remote network
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
PC-B’s NIC receives the frame and ignores the rest of the frame after determining that the destination MAC address of 00-0D does not match it’s own MAC address of 00-0B

17. Destination IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D X A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0D
Source MAC
00-0A
Type
Data
Destination IP address on a remote network
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
The frame is also flooded out S1’s port 4 which is connected to switch S2
S2 receives the frame and examines the source MAC address
The source MAC address of 00-0A and port 1 is in its MAC address table, so it has nothing new to learn
S2 refreshes the 5 minute refresh timer for this entry

18. Destination IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D X A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0D
Source MAC
00-0A
Type
Data
Destination IP address on a remote network
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
S2 can now forward the frame by examining the destination MAC address
The destination MAC address 00-0D is not in its MAC address table, so it floods it out all ports
Again, this is known as an unknown unicast

19. Destination IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D X X A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0D
Source MAC
00-0A
Type
Data
Destination IP address on a remote network
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
PC-C’s NIC and the Router’s NIC receives the frame.
PC-C’s MAC addresses does not match the destination MAC address so it drops the rest of the frame
The router’s MAC address does match the destination MAC address so it copies in the rest of the frame
The router will de-encapsulate the data from the Ethernet frame and process the Layer 3 PDU, the IP packet, for routing.

20. Source IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0A
Source MAC
00-0D
Type
Data
Source IP address on a remote network
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
The router now sends the frame to PC-A
The frame is received by switch S2

21. Source IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B 4
00-0D
Internet S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0A
Source MAC
00-0D
Type
Data
Source IP address on a remote network
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
Switch S2 sees if it has something to learn by examining the source MAC address
The source MAC address is not in its MAC address table
S2 adds the source MAC address 00-0D and the incoming port 4 to its MAC address table

22. Source IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B 4
00-0D
Internet 4
00-0D S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0A
Source MAC
00-0D
Type
Data
Source IP address on a remote network
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
S2 can now forward the frame by examining the destination MAC address
The destination MAC address 00-0A is in its MAC address table, so it filters the frame by sending it out only port 1

23. Source IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B 4
00-0D
Internet 4
00-0D S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0A
Source MAC
00-0D
Type
Data
Source IP address on a remote network
FCS
For every Ethernet frame that enters a port, the switch will:
Step 1: Learn (Building the MAC address table)
Switch learns by examining the source MAC address
If not in MAC address table, then add it along with incoming port number
If in MAC address table, reset 5 minute timer
Switch S1 sees if it has something to learn by examining the source MAC address
The source MAC address is in its MAC address table, so it has nothing new to learn
S1 refreshes the 5 minute refresh timer for this entry

24. Source IP address on a remote network
S1 MAC Address Table
S2 MAC Address Table
Port
MAC Address
Port
MAC Address 1
00-0A 1
00-0A 3
00-0B 4
00-0D
Internet 4
00-0D S1 1 2 3 4 S2 1 2 3 4
Router 1 2
MAC
00-0D A B C
MAC
00-0A
MAC
00-0B
MAC
00-0C
Destination MAC
00-0A
Source MAC
00-0D
Type
Data
Source IP address on a remote network
FCS
Step 2: Forwarding the frame (Using the MAC address table)
Switch forwards by examining the destination MAC address
If broadcast/multicast MAC address, flood out all ports except the port it came in on
If unicast MAC address is in MAC address table, send it out only that port
If unicast MAC address is not in in MAC address table, flood out all ports except the port it came in on (unknown unicast)
S1 can now forward the frame by examining the destination MAC address
The destination MAC address 00-0A is in its MAC address table, so it filters the frame by sending it out only port 1

25. 5.3.2.3 - ARP Operation - ARP Request

26. B A C Internet R1 IP Packet Source IP 192.168.1.110 Destination IP
MAC 00-0B B A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
IP Packet
PCA at has an IPv4 packet to send to
Comparing its IPv4 address with the destination IPv4 address, PCA determines this device is on the same IP network
Ethernet Header
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

27. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
IP Packet
PCA checks its ARP table for but no match
Ethernet Header
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

28. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
PCA puts the IPv4 packet on hold and creates an ARP Request with
Target IPv4 =
Target MAC – unknown
Source MAC 00-A
Destination MAC = broadcast
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

29. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
PCA sends the ARP request which is received by the switch
Because the destination MAC is a broadcast the switch sends it out all ports but the incoming port
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

30. IPv4 Address MAC Address
The target IPv4 is not me.
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
PCB compares the target IPv4 address to its own MAC address and because it is not a match ignores the ARP request
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

31. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
The target IPv4 is not me.
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
Router R1 compares the target IPv4 address to its own MAC address and because it is not a match ignores the ARP request.
Routers do not forward broadcasts.
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

32. IPv4 Address MAC Address
MAC 00-0B
The target IPv4 is me! B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
The target address in the ARP request matches PCC’s IPv4 address
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

33. 5.3.2.4 - ARP Operation - ARP Reply

34. IPv4 Address MAC Address
MAC 00-0B
The target IPv4 is me! B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
The target address in the ARP request matches PCC’s IPv4 address
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

35. Here is my MAC address for the IPv4 address you were looking for!
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
Ethernet Header
ARP Reply
Destination MAC
00-0A
Source MAC
00-0C
Sender IPv4
Sender MAC
00-0C
PCC sends an ARP reply to the sender of the ARP request
This is a unicast
Sender IPv4 =
Sender MAC – 00-0C
Source MAC 00-0C
Destination MAC = 00-0A
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

36. Here is my MAC address for the IPv4 address you were looking for!
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
Ethernet Header
ARP Reply
Destination MAC
00-0A
Source MAC
00-0C
Sender IPv4
Sender MAC
00-0C
PCA receives the ARP reply
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

37. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address C A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
Ethernet Header
ARP Reply
Destination MAC
00-0A
Source MAC
00-0C
Sender IPv4
Sender MAC
00-0C
PCC Adds the sender’s IPv4 address and MAC address to its ARP table
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

38. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address C A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
PCC takes the IPv4 packet off hold
PCC uses the MAC address for the destination MAC address
Ethernet Header
IP Packet
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

39. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address C A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Internet R1
PCA now sends this Ethernet frame with the encapsulated IPv4 packet out its Ethernet NIC
Ethernet Header
IP Packet
Destination MAC
00-0C
Source MAC
00-0A
Source IP
Destination IP

40. 5.3.2.5 - ARP Role in Remote Communication

41. B A C Internet R1 IP Packet Source IP 192.168.1.110 Destination IP
MAC 00-0B B A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
PCA at has an IPv4 packet to send to
Comparing its IPv4 address with the destination IPv4 address, PCA determines this device is on a different IP network
PCA must sent this packet to its default gateway
Ethernet Header
IP Packet
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

42. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
PCA checks its ARP table for but no match
Ethernet Header
IP Packet
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

43. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
PCA puts the IPv4 packet on hold and creates an ARP Request with
Target IPv4 =
Target MAC – unknown
Source MAC 00-A
Destination MAC = broadcast
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

44. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
PCA sends the ARP request which is received by the switch
Because the destination MAC is a broadcast the switch sends it out all ports but the incoming port
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

45. IPv4 Address MAC Address
The target IPv4 is not me.
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
PCB compares the target IPv4 address to its own MAC address and because it is not a match ignores the ARP request
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

46. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address
The target IPv4 is not me. A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
PCC compares the target IPv4 address to its own MAC address and because it is not a match ignores the ARP request
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

47. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
The target IPv4 is for me!
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
Ethernet Header
ARP Request
Destination MAC
FF-FF
Source MAC
00-0A
Target IPv4
Target MAC
???
The target address in the ARP request matches the router’s the IPv4 address on its G0/0 interface
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

48. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
Here is my MAC address for the IPv4 address you were looking for!
MAC 00-0D
Default Gateway:
Internet R1
Ethernet Header
ARP Reply
Destination MAC
00-0A
Source MAC
00-0D
Target IPv4
Target MAC
00-0D
PCC sends an ARP reply to the sender of the ARP request
This is a unicast
Sender IPv4 =
Sender MAC – 00-0D
Source MAC 00-0D
Destination MAC = 00-0A
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

49. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
Ethernet Header
ARP Reply
Destination MAC
00-0A
Source MAC
00-0D
Target IPv4
Target MAC
00-0D
PCA receives the ARP reply
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

50. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address D A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
Ethernet Header
ARP Reply
Destination MAC
00-00A
Source MAC
00-0D
Target IPv4
Target MAC
00-0D
PCC Adds the sender’s IPv4 address and MAC address to its ARP table
Ethernet Header
IP Packet
On Hold
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

51. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address D A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
PCC takes the IPv4 packet off hold
PCC uses the MAC address for the destination MAC address
Ethernet Header
IP Packet
Destination MAC
???
Source MAC
00-0A
Source IP
Destination IP

52. IPv4 Address MAC Address
MAC 00-0B B
PC-A’s ARP Cache
IPv4 Address MAC Address D A C
MAC 00-0C
MAC 00-0A
MAC 00-0D
Default Gateway:
Internet R1
PCA now sends this Ethernet frame with the encapsulated IPv4 packet out its Ethernet NIC towards the default gateway
Ethernet Header
IP Packet
Destination MAC
00-0D
Source MAC
00-0A
Source IP
Destination IP