I've gotten confused when reading about encapsulation and the OSI and TCP/IP models. I first read about message formatting and encapsulation and the diagram I read showed:

Destination | Source

and these are the frame addressing

and this is followed by the encapsulated message which involves:

Start message indicator| Receiver | Sender | Encapsulated bits

and then ends with the CRC.

I understand that but then I read about OSI and TCP/IP model. But where does all of what I said above (message formatting encapsulation) come into? I know there are different layers but these are different to the format I just mentioned, they apply each header as they pass the PDU down the stack. I'm just confused at when the format I explained above is used.

1 Answer 1


Most of the time, "encapsulation" is the prepending of a layer header.

  • TCP prepends a TCP header to the application payload.
  • IP prepends an IP header to the TCP segment.
  • Ethernet data link player prepends an Ethernet frame header and appends a checksum (FCS).
  • Ethernet physical layer prepends its preamble and appends IPG.

In detail, TCP over IPv4 over Ethernet with maximum payload looks like this enter image description here

Of course, there are other protocols than TCP, IPv4 and Ethernet for layers 4, 3, 2, and 1.

  • But I'm confused by Ethernet because some diagrams are showing that ethernet uses destination MAC, source MAC, preamble, Receiver IP Address, Sender IP address, user data and the trailer. Whereas others show that Ethernet only uses MAC Addresses, not IP Addresses
    – The_Bear
    Commented Mar 3, 2019 at 22:11
  • 1
    The IP addresses and user data are encapsulated upper layer protocols. In Ethernet all of that is “payload” or data.
    – Ron Trunk
    Commented Mar 4, 2019 at 1:01
  • @The_Bear Check the diagram: each layer only sees the elements in its own row, the rest is either payload or overhead (overhead is usually completely invisible except for the channel occupation).
    – Zac67
    Commented Mar 4, 2019 at 7:17

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