Layered models give you a shared vocabulary for where a problem or feature lives. The OSI (Open Systems Interconnection) model has seven layers: 1 Physical (bits on the wire, cables, optics), 2 Data Link (frames, MAC addresses, switches), 3 Network (packets, IP addresses, routers), 4 Transport (segments, TCP and UDP ports), 5 Session, 6 Presentation and 7 Application. The TCP/IP model used by the real internet collapses these into four: Network Access (or Link), Internet, Transport and Application, where Application covers OSI layers 5 to 7.
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