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The OSI Model

In brief

The seven-layer OSI model — the shared map everyone uses to say where a network thing happens. You will use its layer numbers for the rest of your career.

Objectives

  • Name the seven OSI layers in order
  • Locate IP, TCP and HTTP on the stack
  • Explain the down-then-up path of data

The OSI model describes a network as seven layers, from the physical wire up to the application you use. It is a map: no real system implements it exactly, but everyone uses its vocabulary to talk about where things happen.

7ApplicationHTTP, DNS6PresentationTLS, encoding5Sessionconnections4TransportTCP, UDP3NetworkIP, routing2Data linkEthernet, MAC1Physicalcables, radio
The seven OSI layers. Data travels down the stack on the sender and up on the receiver.

Reading the stack

Data starts at the top, on your application, and travels down the stack as each layer adds its header. It crosses the wire, then travels back up the stack on the other machine, each layer stripping the header its counterpart added.

The layers you will meet most as a security learner are 3 (network, where IP lives), 4 (transport, where TCP and UDP live), and 7 (application, where HTTP lives).

Note

People say "that is a layer-7 problem" or "a layer-2 attack" constantly. The number is just shorthand for where in this stack something happens.

Recap

  • OSI has seven layers, physical at the bottom, application at the top
  • Data goes down the stack, across, then up the other side
  • Layers 3, 4 and 7 matter most for security