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As I understand it:

A switch (layer 2) seems to define a LAN and a router (layer 3) enables communication between distant LANs.

A device on a LAN will give it's MAC address to the switch upon broadcast, which passes it off to the router to assign an IP address to that connection before sending its message across the internet to the router of the distant LAN. So the message travels across the physical layer (layer 1) and reaches the destination router (layer 3) which converts the recipient IP address into a MAC address for the switch (layer 2) so the correct device receives it.

I am confused about the picture(s) I painted above -- it seems that the OSI model is out of sequence. We move from layers 1 to 3 to 2. What am I missing? Can the OSI model occur out of sequence?

newcomer-91
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  • The OSI model is just a model, and nothing in the real world has actually implemented it. The IP Services model is much closer to reality, but your understanding of what actually happens is flawed. See the above linked question and answers for a detail of what really happens. – Ron Maupin Oct 19 '20 at 14:03
  • @RonMaupin What it the "IP Services model"? Do you mean TCP/IP Protocol Stack? – Albin Oct 19 '20 at 18:40
  • @Albin, the TCP/IP name is misleading because it also applies to protocols other than TCP. It revolves around IP, and it can use any of the transport protocols, such as TCP, UDP, SCTP, etc. – Ron Maupin Oct 19 '20 at 18:49
  • Ah, ok, but "IP Services model" is not an "official" term, right? (I couldn't find it anywhere but your comments) – Albin Oct 19 '20 at 19:28
  • @Albin, I believe I got that from Vint Cerf (the father of IP) at a Global IPv6 conference, – Ron Maupin Oct 19 '20 at 20:42
  • The official term is Internet Protocol Suite, described in RFC 1122. – Ron Trunk Oct 19 '20 at 22:56
  • @RonMaupin Just FYI, there have been several full implementations of the OSI stack, from ACSE to 802.3. (Not that anyone cares anymore.) At least one OSI application-layer protocol suite, X.500, the OSI directory services, is still in wide use for things like DNS and telephony. Presumably they found ways to bypass the worst aspects of the upper layers! – Jeff Learman Dec 18 '20 at 02:35
  • The OSI model often appears out of order, though not as in your example. For out-of-order examples see https://networkengineering.stackexchange.com/questions/17426/why-pptp-l2tp-pppoe-are-in-osi-layer2/49935#49935 . – Jeff Learman Dec 18 '20 at 02:37

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