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Explore the process packets is moving in the network

Explore the process packets is moving in the network

We explore the process of a PC (Pc0) contact (ping) to the PC in the network (pc2), Howto the computer works?

Part 1: The machine on the same network 1

mo hinh mang LAN

 Figure 1: Topo network

1. The machine performed PC0

Ping 172.16.1.3

Ping uses ICMP protocol operates at Layer 3 (network) of the OSI model, while the machines want to transfer data to another MAC address, so how to know the MAC address of the machine is the 172.16.1.3 much to the media now?

Meanwhile, PC0 pause the ICMP protocol, and conduct initialization protocol ARP request to find the MAC address of the IP 172.16.1.3 as follows

arp request

Figure 2: The fields in the ARP Request

Source MAC: The MAC address of Pc0

Target MAC: is waiting to find the owner of the IP 172.16.1.3 is to fill up the values ​​= 0

(Great attention is the PC with the IP address = 172.16.1.3 is sent back to my MAC address)

After initializing the ARP request packet is given to Layer II (data link) to conduct the frame before moving into the bits sent over wire in Layer I (physical), the frame has the following values:

frame broadcast

Figure 3: The field in the Frame

ARP Request packet is added to the header datalink layer with the

SRC MAC: Still keep moving down the value of ARP (MAC’s is PC0)

DEST MAC (MAC pc2, unknown to PC0 converted FFFF.FFFF.FFFF (this value is called the broadcast address of the Layer 2), why Frame conduct this value, because when the switch receives a frame the target values FFFF.FFFF.FFFF, Switch ​​will be sent to all the other port Switch and all the computers are plugged into the switch will be heard by the machine frame Pc0 promotion.

– And the frame is moved down one layer to move into the bit (0.1) and signaling network card’s signal transmission medium (such as electricity, the 0,5v, how many optical wavelengths, radio, the frequencies much …) and is sent to Switch

2. The device Switch

SWitch receives the signal on line is 0v, 5v corresponding bit 0.1 and converted into the bit sequence, and converted to Frame (for SW work at Layer 2, ie working with frame)

Switch check dest MAC address (the destination MAC address of the frame to be sent), when it saw the value FFFF.FFFF.FFFF SWitch (the broadcast) immediately moved Frame and send out all Port are the Pc Connection

3. At the computer

The computer receives the bit and moved into reading frame and ARP Request packet contents, whereas the PC compares the destination IP address with the IP of the PC.

The PC1, PC3 will delete the IP packet is not equal because the ARP Request.

Private IP = PC2 with the destination IP ARP Request to conduct initialization ARP Reply packet to send to PC0, APR reply packet contents as follows:

frame filed

Figure 4: The contents of the ARP Reply

In the image above we see:

Source IP = 172.16.1.3 is PC2

Target IP = 172.16.1.1 is PC0

Soucre’s MAC MAC = 0010.1150.596D of PC2

Target MAC: MAC’s 0002.4AB7.2146 of PC1

And ARP reply is sent to close the frame and through the switch and to be Pc0,

4. At PC0

– Now PC0 received MAC of PC2 and store information on ARP Cache pc2

And later want to change the MAC IP 172.16.1.3 PC0 just look in the ARP cache table, of course restart the PC is no longer the table information.

arp cache

Figure 5: Table temporarily store the corresponding IP and Mac

– PC0 conducted the ICMP protocol has been stopped earlier and perform Ping

Part 2: Routing on a Router (continue)

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About Nguyễn Thanh Sơn

Nguyễn Thanh Sơn
Network Security, Web Design, Computer Science

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