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According to the OSI model, data is encapsulated from layer 7 down to layer 1.

Since normally a setup would be

PC(layer7, 6, 5, 4) > switch(layer 2) > Router(layer 3) > Modem(layer 1)

it seems to me like either layer 2 header is being added before layer 3, or the router would send data backward to the switch to get the layer 2 header? Am I missing something or misunderstanding the interactions between the devices?

Thanks

A Lone Loser
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    In don't exactly get the question. Layer 2 knows MAC addresses only. Layer 3 knows IPs. To get the MAC address to send a package to, the ARP protocol is used. It is like dns for IPs and returns the correct MAC address. – HEllRZA Jul 09 '22 at 00:26
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    Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Jul 09 '22 at 14:14

1 Answers1

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It might be little easier to understand if you think about the process of building an Ethernet Frame containing an IP Packet that is destined for something off the network:

Computer with IP 10.10.10.1 with a mask of 255.255.255.0 and a default gateway of 10.10.10.254. And lets say its mac address (fake) is 12:34:56:78:90:12

Lets say the default gateway 10.10.10.254, has a mac (also fake) of 22:33:44:22:33:44

So, now the computer wants to send traffic to an IP 50.50.50.50. It knows from its IP address and mask that the IP is off network, so:

  1. It sends out an ARP request for IP 10.10.10.254
  2. It gets back a response with the mac 22:33:44:22:33:44

Now it builds an ethernet frame:
From MAC: 12:34:56:78:90:12
To MAC: 22:33:44:22:33:44 (this mac is local)

With an IP Packet encapsulated inside:
From IP: 10.10.10.1
To IP: 50.50.50.50 (this ip is remote)

So, the real packet isn't sent until it is built. The ARP process helps to get the entire packet built. If that destination IP had been local, the same process would have taken place, except it would ARP for the local IP's MAC Address and the To MAC would have been that response and the destination IP would be local (like 10.10.10.50).

All of the communications between devices really happens at layer 2, via the MAC addresses, or Broadcast Mac Address (like in the case of ARP). The IP packets are delivered via those Layer 2 frames.

LGGJR
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