3.4 KiB
id, aliases, tags
| id | aliases | tags | |
|---|---|---|---|
| 1777801183-SYMV |
|
The Life of a Packet
Network Topology
We will follow a packet traveling from PC1 to PC4 across multiple routers.
Devices and MAC Addresses
| Device | Interface | MAC Address |
|---|---|---|
| PC1 | - | 1111 |
| R1 | G0/2 | aaaa |
| G0/0 | bbbb | |
| R2 | G0/0 | cccc |
| G0/1 | dddd | |
| R4 | G0/0 | eeee |
| G0/1 | ffee | |
| PC4 | - | 4444 |
IP Addresses
- Source IP: 192.168.1.1
- Destination IP: 192.168.4.1
Step 1: PC1 Needs a Gateway
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.
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.
ARP Request (Broadcast)
- Source IP: 192.168.1.1
- Destination IP: 192.168.1.254
- Source MAC: 1111
- Destination MAC: ffff.ffff.ffff (broadcast)
“Who has 192.168.1.254? Tell me!”
ARP Reply (Unicast)
Router R1 responds:
- Source IP: 192.168.1.254
- Destination IP: 192.168.1.1
- Source MAC: aaaa
- Destination MAC: 1111
Now PC1 knows the MAC address of its gateway.
Step 2: PC1 Sends the Packet to R1
PC1 builds the frame:
+-----------------+--------+
| SRC: 192.168.1.1 | DST: aaaa |
| DST: 192.168.4.1 | SRC: 1111 |
+-----------------+--------+
- The IP header stays constant end-to-end
- The MAC addresses are only for the local hop
The packet is sent to R1.
Step 3: R1 Forwards to R2
R1 checks its routing table:
| Destination | Next Hop |
|---|---|
| 192.168.4.0/24 | 192.168.12.2 |
R1 must forward the packet to R2, but first it needs R2’s MAC address.
ARP Process (again)
- R1 sends ARP request
- R2 replies with MAC cccc
New Frame (re-encapsulation)
+-----------------+--------+
| SRC: 192.168.1.1 | DST: cccc |
| DST: 192.168.4.1 | SRC: bbbb |
+-----------------+--------+
Notice:
- IP addresses are unchanged
- MAC addresses are updated for the new hop
Step 4: R2 Forwards to R4
R2 receives the packet, checks its routing table, and determines the next hop is R4.
Same process:
- ARP request
- ARP reply from R4 (MAC: eeee)
New Frame
+-----------------+--------+
| SRC: 192.168.1.1 | DST: eeee |
| DST: 192.168.4.1 | SRC: dddd |
+-----------------+--------+
Step 5: R4 Delivers to PC4
R4 sees that the destination network is directly connected.
It performs ARP to find PC4’s MAC:
- ARP request
- ARP reply from PC4 (MAC: 4444)
Final Frame
+-----------------+--------+
| SRC: 192.168.1.1 | DST: 4444 |
| DST: 192.168.4.1 | SRC: ffee |
+-----------------+--------+
Final Insight
This journey hides a simple but powerful rule:
- IP addresses = end-to-end identity (never change)
- MAC addresses = hop-by-hop delivery (change every step)
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.
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. 🚀