1.1The OSI reference model and TCP/IP layers

NW's exam A-2 does not test rote memorization such as "the OSI reference model has 7 layers"; it tests the ability to isolate which layer a communication failure is occurring at and judge the next diagnostic step. Even a single symptom like "the web page will not load" could stem from a severed cable (physical layer), a routing misconfiguration (network layer), or a failed DNS lookup (application layer). The layer model functions as a map for fault isolation.

1.1.1The seven layers of the OSI reference model

• The OSI reference model is an international standard design model dividing communication functions into seven layers: from bottom to top, the physical layer (L1), data link layer (L2), network layer (L3), transport layer (L4), session layer (L5), presentation layer (L6), and application layer (L7). Each layer uses only the service provided by the layer directly below it and provides a service to the layer directly above it (layer independence).
• Layer independence means swapping a lower layer does not affect upper layers (e.g., switching the physical medium from Ethernet to Wi-Fi leaves IP/TCP applications unaffected). This property is exactly why you can investigate "which layer the failure occurred at" independently of the other layers.

1.1.2Correspondence with the TCP/IP four-layer model

• The TCP/IP layer model is the four-layer model used in practical internet communication: the network interface layer (roughly L1+L2), internet layer (roughly L3), transport layer (roughly L4), and application layer (roughly L5-L7). The key practical difference is that it collapses OSI's top three layers (session, presentation, application) into one.
• Knowing this correspondence lets you immediately translate device catalog specs written in OSI layer numbers—such as "L2 switch" or "L3 switch"—into their TCP/IP implementation (Ethernet = L1+L2, IP = the internet layer).

1.1.3Encapsulation and PDUs

• Encapsulation is the process by which, on the sending side, each layer adds its own header (and sometimes a trailer) to the data handed down from the layer above, then passes it to the layer below. On the receiving side, the reverse process, decapsulation, strips each header away layer by layer.
• The unit of data at each layer is called a PDU (Protocol Data Unit), and its name differs by layer: the application layer calls it data, the transport layer calls it a segment (TCP) or datagram (UDP), the network layer calls it a packet, and the data link layer calls it a frame. In a fault report, the difference in wording—"packets aren't arriving" versus "frames are corrupted"—is actually a hint about which layer the problem was observed at.

Suppose you are the operator of an internal corporate network and receive a report that "a specific business system, and only that system, cannot be reached" from one site. First, suspecting L3 (the network layer), you check reachability to the target server's IP address with ping and get a response, letting you conclude that IP reachability is normal. Next, you find that accessing it via web browser times out, while connectivity to a different port on the same server is fine—raising the possibility that a specific L4 port (e.g., TCP 443) is being blocked somewhere along the path, an L4 (transport layer) issue. It would be premature here to conclude "ping succeeds, so the network must be fine": ping (ICMP) only verifies L3 reachability and cannot detect L4 port-level filtering such as a firewall ACL. Investigating further reveals that a firewall along the path was blocking only traffic destined for TCP 443. If the root cause had instead been a failed DNS lookup (the internal system's hostname could not be resolved), that would be an L7 (application layer) issue requiring an entirely different remedy (checking the DNS server configuration). This kind of stepwise isolation that forms a hypothesis about "which layer" from the symptom is far more efficient incident response than blindly changing configuration.

1.1.4Section summary

• The OSI 7 layers (physical/data link/network/transport/session/presentation/application) collapse L5-L7 into a single application layer in the TCP/IP 4-layer model
• Encapsulation is the process of each layer adding a header before passing data down. PDU names differ by layer: data/segment/packet/frame
• Fault isolation means forming a hypothesis about which layer a symptom belongs to and verifying it step by step (ping only checks L3; L4/L7 require separate checks)