I am not sure of any accepted answer but one reasoning for this separation would be for applications that only implement up to one layer.
If there is a router that only implements up to the internet layer(because it doesn't need port control up to that point) it would easily be able to implement this functionality up the internet layer only.
This also proves the same for software that would only implement up to the network layer such as old switches
I agree with this. I'd also expand on this and say that different devices "unwrap" different layers of a packet. So yeah, it wouldn't necessarily make sense for an edge router to care what port is needed. It just needs to know where to send the packet (IP) and the target machine handles the rest.
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I am not sure of any accepted answer but one reasoning for this separation would be for applications that only implement up to one layer.
If there is a router that only implements up to the internet layer(because it doesn't need port control up to that point) it would easily be able to implement this functionality up the internet layer only.
This also proves the same for software that would only implement up to the network layer such as old switches
I agree with this. I'd also expand on this and say that different devices "unwrap" different layers of a packet. So yeah, it wouldn't necessarily make sense for an edge router to care what port is needed. It just needs to know where to send the packet (IP) and the target machine handles the rest.