CISSP 11.1 - OSI Model (Part 1 of 2)

This episode of the ISC2 Certified Information Systems Security Professional (CISSP) exam prep series opens Domain 4 by unpacking the reference model that describes how two machines talk. It builds the mental map behind every network conversation — the shared vocabulary that lets you place a problem instantly, whether a vendor claims layer-specific filtering or your team is triaging an outage.

Frequently Asked Questions

Why was the OSI reference model created?

Rules that govern how data crosses a wire are called protocols, and early on every vendor built its own, so their gear refused to cooperate. A standards body designed one shared blueprint so systems from different makers could interoperate. Treat it as a theoretical ideal, not a literal build — it describes how communication should work on perfect hardware, and it became the common reference everyone points to.

How do the seven layers fit together?

The model splits the whole job of communication into seven stacked layers, and each layer owns one narrow set of tasks and nothing more. A layer only ever talks to the one directly above it and the one directly below it, like an assembly line where each station does a single step. Data flows down the stack on the sending side and back up on the receiving side.

What are encapsulation and deencapsulation?

Encapsulation is how data gets wrapped as it travels down the stack: each layer adds its own header, and sometimes a footer, around what it received from above, and the whole bundle becomes the payload of the layer below. On the far end each layer peels off its own wrapper, which is called deencapsulation. Each header is a private note readable only by the identical layer on the other machine, known as peer-layer communication.

What is the network container called at each layer?

At the top three layers it is simply a protocol data unit. At the transport layer it becomes a segment, or a datagram if the connectionless protocol is used. At the network layer it is a packet, at the data link layer it is a frame, and at the physical layer it is nothing but bits on the wire. The exam uses these names to tell you which layer it is testing.

What does the application layer actually do?

It is the doorway between your software and the stack. The program itself does not live here — what lives here are the protocols and services that let software transfer files, exchange messages, and reach remote systems. Think of it as the reception desk of a building: it does not do the work inside, but nothing gets in or out without passing through it first.

Reference: This article is based on concepts discussed in CISSP 11.1 - OSI Model (Part 1 of 2).