Update 12. The Life of a Packet.md
the life of a packet final version
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We will follow a packet from PC1 to PC4
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We will follow a packet traveling from **PC1** to **PC4** across multiple routers.
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mac address for the machine
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### Devices and MAC Addresses
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PC 1 -> 1111
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| Device | Interface | MAC Address |
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R1 -> G0/2 aaaa -> G0/0 -> bbbb
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| ------ | --------- | ----------- |
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R2 -> G0/0 cccc -> G0/1 -> dddd
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| PC1 | - | 1111 |
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R4 -> G0/0 eeee -> G0/1 -> ffee
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| R1 | G0/2 | aaaa |
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PC 4 -> 4444
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| | G0/0 | bbbb |
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| R2 | G0/0 | cccc |
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| | G0/1 | dddd |
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| R4 | G0/0 | eeee |
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| | G0/1 | ffee |
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| PC4 | - | 4444 |
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SRC: 192.168.1.1
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### IP Addresses
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DST: 192.168.4.1
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PC 1 is in the 192.168.1.0/24 network it need to send the packet in another network
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* **Source IP:** 192.168.1.1
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* **Destination IP:** 192.168.4.1
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First PC will use An ARP request
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---
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it need to find the next interface for the sending the packet
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SRC IP: 192.168.1.1
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## Step 1: PC1 Needs a Gateway
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DST IP: 192.168.1.254 -> gateway
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DST MAC: ffff.fff.fff -> all of the interface
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Src MAC: 1111
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ARP request = Broadcast
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PC1 belongs to the **192.168.1.0/24** network, while the destination is in a different network. That means the packet must be sent to the **default gateway**.
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ARP requply = UNICAST
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R1 will answer with an ARP Reply
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Before sending anything, PC1 needs the MAC address of the gateway (192.168.1.254). It doesn’t have it yet, so it uses ARP.
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SRC IP: 192.168.1.254
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### ARP Request (Broadcast)
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DST IP: 192.168.1.1 -> PC1
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DST MAC: 1111
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Src Mac: aaaa
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Now PC1 now the Mac address of the default gateway and will encapsulated the packet
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* Source IP: 192.168.1.1
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with the internet header
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* Destination IP: 192.168.1.254
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* Source MAC: 1111
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* Destination MAC: ffff.ffff.ffff (broadcast)
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> “Who has 192.168.1.254? Tell me!”
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### ARP Reply (Unicast)
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Router R1 responds:
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* Source IP: 192.168.1.254
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* Destination IP: 192.168.1.1
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* Source MAC: aaaa
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* Destination MAC: 1111
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Now PC1 knows the MAC address of its gateway.
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---
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## Step 2: PC1 Sends the Packet to R1
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PC1 builds the frame:
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```
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+-----------------+--------+
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+-----------------+--------+
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|SRC: 192.168.1.1 |Dst:aaaa|
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| SRC: 192.168.1.1 | DST: aaaa |
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|DST: 192.168.4.1 |Src:1111|
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| DST: 192.168.4.1 | SRC: 1111 |
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+-----------------+--------+
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+-----------------+--------+
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```
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R1 Receive the packet and watch the routing table and see
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* The **IP header** stays constant end-to-end
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* The **MAC addresses** are only for the local hop
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Destination Next Hop
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The packet is sent to R1.
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192.168.4.0/24 192.168.12.2
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R1 will encapsulated the packet for sending to R2
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---
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He will have tot do an ARP request
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R2 send a arp reply
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Same process as above
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## Step 3: R1 Forwards to R2
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R1 checks its routing table:
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| Destination | Next Hop |
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| -------------- | ------------ |
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| 192.168.4.0/24 | 192.168.12.2 |
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R1 must forward the packet to R2, but first it needs R2’s MAC address.
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### ARP Process (again)
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* R1 sends ARP request
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* R2 replies with MAC **cccc**
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### New Frame (re-encapsulation)
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```
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+-----------------+--------+
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+-----------------+--------+
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|SRC: 192.168.1.1 |Dst:cccc|
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| SRC: 192.168.1.1 | DST: cccc |
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|DST: 192.168.4.1 |Src:bbbb|
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| DST: 192.168.4.1 | SRC: bbbb |
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+-----------------+--------+
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+-----------------+--------+
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```
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Notice:
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R2 Receive the packet and watch the routing table
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* IP addresses are unchanged
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R2 will encapsulated the packet for sending to R4
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* MAC addresses are updated for the new hop
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He will have tot do an ARP request
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R4 send a arp reply
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Same process as above
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---
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## Step 4: R2 Forwards to R4
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R2 receives the packet, checks its routing table, and determines the next hop is R4.
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Same process:
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* ARP request
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* ARP reply from R4 (MAC: eeee)
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### New Frame
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```
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+-----------------+--------+
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+-----------------+--------+
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|SRC: 192.168.1.1 |Dst:eeee|
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| SRC: 192.168.1.1 | DST: eeee |
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|DST: 192.168.4.1 |Src:dddd|
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| DST: 192.168.4.1 | SRC: dddd |
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+-----------------+--------+
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+-----------------+--------+
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```
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R4 Receive the packet and watch the routing table
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---
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R4 will encapsulated the packet for sending to PC4
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He will have tot do an ARP request
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PC4 send a arp reply
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Same process as above
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## Step 5: R4 Delivers to PC4
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R4 sees that the destination network is directly connected.
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It performs ARP to find PC4’s MAC:
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* ARP request
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* ARP reply from PC4 (MAC: 4444)
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### Final Frame
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```
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+-----------------+--------+
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+-----------------+--------+
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|SRC: 192.168.1.1 |Dst:4444|
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| SRC: 192.168.1.1 | DST: 4444 |
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|DST: 192.168.4.1 |Src:ffee|
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| DST: 192.168.4.1 | SRC: ffee |
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+-----------------+--------+
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+-----------------+--------+
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```
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---
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## Final Insight
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This journey hides a simple but powerful rule:
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* **IP addresses = end-to-end identity (never change)**
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* **MAC addresses = hop-by-hop delivery (change every step)**
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Each router peels off the old frame and wraps the packet in a new one, like a traveler switching taxis at every city while keeping the same passport.
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And just like that, the packet arrives at PC4, having quietly crossed networks, routers, and multiple layers of logic without ever losing its sense of direction. 🚀
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