Point to point is the simplest computer network in which two devices are directly connected to each other. In this practice, we will see that two devices can be connected directly to create a network by making sure that their network IDs are the same.
In this example, we will create a /30 network with following two IP addresses:
10.10.10.1/30
10.10.10.2/30
The subnet mask for /30 is 255.255.255.252.
Step 1. In Packet Tracer, pull two PCs in the canvas:
Step 2. Connect the two PCs using the options Connections and then Automatically Choose Connection Type.
Step 3. Click on the PCs and go to Desktop and then IP Configuration.
Step 4. Assign IP address to first PC.
Step 5. Assign IP address to second PC.
Finally, we create following network:
In this case, both devices have the same network ID 10.10.10.0.
Hence, when we send a simple PDU from one PC to another, it is successfully sent as shown below:
At the bottom right, the status column will show Successful, which means the simple PDU could successfully move between the two PCs suggesting connectivity.
Network protocols
- ICMP
- ARP
ICMP stands for Internet Control Message Protocol. It is used for diagnostic purposes during network troubleshooting. Used when using the ping command to test network connectivity. Ping command uses source and destination IP addresses.
ARP stands for Address Resolution Protocol. Communication in a computer network only works when the physical address or MAC address of the destination device is known. In other words, packet delivery is the task of layer 2 of the OSI model. Hence, ARP is used to find out the MAC address of the destination device from its IP address.
Protocol Analysis using Simulation
We can see the working of these two protocols by using Packet Tracer’s simulation feature.
Step 1. Switch to Simulation from Realtime.
Step 2. Click on the Show All/None button to remove all the visible events.
Step 3. Click on Edit Filters
Step 4. Select ICMP and ARP
Step 5. Select a simple PDU to send from the left PC to the right PC.
Note: PDU stands for Protocol Data Unit. It is a unit of data which can be transmitted between the devices in a network.
Step 6. Click on the left PC to select it as the source of PDU and then click on the right PC to select it as the receiver of the PDU.
The moment we select the PC as receiver, we will see the following:
Now, we can see how ICMP and ARP protocols work.
Note: If you only see the ICMP PDU, it means ARP has already done its work. To see ARP in action, make sure you reset the network before going into simulation mode.
To reset the network do the following:
Step 1. Switch to Realtime mode.
Step 2. Click on the Power Cycle Devices button.
Step 3. Continue with sending PDU in simulation mode as earlier.
So here, we can see that there are two PDUs. One is for ICMP and another one is for ARP.
By clicking on the forward button, we can see how these protocols work.
Event details:
- ICMP PDU is created with following details:
Source IP: 10.10.10.1
Destination IP: 10.10.10.2
Based on the subnet mask calculation, it is identified that the destination device is in the same network. Hence, the next-hop address is set as the destination IP address itself. Next-hop address refers to the address where the PDU needs to go to reach its final destination.
In order for PDU to reach the destination, the destination’s MAC address has to be found out. The sender checks its ARP table to search for the MAC address.
For the first time, the destination’s MAC address is not found in the table. Hence, an ARP request is initiated.
ARP PDU is created with following details:
Sender IP address: 10.10.10.1
Sender MAC address: 0030.A337.94C9
Destination IP address: 10.10.10.2
Destination MAC address: FFFF:FFFF:FFFF
It should be noted that the ARP request uses a special MAC address to send the PDU to all the devices in the network. This address is FFFF:FFFF:FFFF.
- ARP PDU arrives at the second PC
The second PC sees that its IP address matches the destination IP address in ARP PDU. Thus the second PC receives the ARP PDU and inserts its MAC address in the PDU.
- ARP PDU returns to the first PC
The PDU has following details:
Sender IP address: 10.10.10.2
Sender MAC address: 000D.BD8C.C168
Destination IP address: 10.10.10.1
Destination MAC address: 0030.A337.94C9
Thus we can see that the ARP PDU has the second PC’s MAC address.
Now both PCs have each other’s MAC address.
- ICMP PDU reaches the second PC
ICMP PDU now shall include details regarding the sender and receiver’s IP and MAC addresses. Hence, the ICMP PDU arrives at the second PC.
Note: It should be noted that, while sending the PDU, layer 3 details are encapsulated inside layer 2 details.
- ICMP PDU returns to the first PC from the second PC
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