OSI to TCP/IP mapping and layer terminology
Two models, one winner
Developed by different organizations around the same time, both aiming to organize how data transmission works. The TCP/IP-based internet took off so fast that OSI protocol development and adoption fell behind.
Almost no OSI-spec protocols are in wide use today, but the seven-layer model itself shaped protocol and product design everywhere.
Layer mapping
- OSI 7 Application, 6 Presentation, 5 Session map to TCP/IP Application: supporting applications talking to the lower layers.
- OSI 4 Transport maps to TCP/IP Transport: multiple host applications access the network, best-effort or reliable delivery.
- OSI 3 Network maps to TCP/IP Internet: addressing and routing between devices.
- OSI 2 Data Link and 1 Physical map to TCP/IP Link: interfacing with network hardware, accessing the media, hardware addresses.
Why everyone uses OSI numbers
OSI layer functions are clearly defined, so devices and protocols get named by OSI layer even though the protocols themselves are TCP/IP:
- A Layer 2 switch is a LAN switch. “Layer 2” means OSI data link, and everyone knows the function set that implies.
- IP is a “network layer protocol” or “Layer 3 protocol” (TCP/IP’s internet layer = OSI network layer).
- TCP and UDP are “Layer 4 protocols”: same functions as the OSI transport layer, different specific protocols.
- A Layer 3 switch is a switch (traditionally L2 only) that can also do L3 internet-layer work, which was traditionally router territory.
In practice the industry mixes them: OSI terminology and layer numbers, TCP/IP protocols.