OSI model: learn the 7 layers interactively

The OSI model is an ISO reference model (ISO/IEC 7498-1). OSI stands for Open Systems Interconnection. It divides communication between two computers into seven layers, from transmitting bits over a cable to the application. Each layer has a clear task and uses only the services of the layer below it. This helps you understand and plan networks and pinpoint faults.

The OSI model has seven layers. From bottom to top: 1 Physical, 2 Data Link, 3 Network, 4 Transport, 5 Session, 6 Presentation, and 7 Application. Layers 1 to 4 are transport-oriented; layers 5 to 7 are application-oriented.

The OSI model has seven layers. It is a vendor-neutral ISO reference model now used mainly for learning and troubleshooting. The TCP/IP model has four layers and describes the protocols the internet uses: network access (OSI 1 and 2), internet (OSI 3), transport (OSI 4) and application (OSI 5 to 7). Session and presentation are part of the application layer in this model. The tool shows both models side by side with their layers aligned.

A switch operates at layer 2 because it forwards frames based on MAC addresses. A router operates at layer 3 and determines the route based on IP addresses. Hubs and repeaters operate at layer 1; they only relay signals. A Layer 3 switch can do both: it switches at layer 2 and routes at layer 3. VLANs (IEEE 802.1Q) belong to layer 2 because the VLAN ID is a tag in the Ethernet frame.

Layer 1 transmits bits, Layer 2 frames, Layer 3 packets, and Layer 4 segments for TCP or datagrams for UDP. At Layers 5 to 7, they are simply called data. When sending, layers add their headers to the front; Layer 2 also adds a checksum (FCS) as a trailer. This is called encapsulation.

Both are borderline cases. ARP links an IP address (Layer 3) to a MAC address (Layer 2), but travels directly in an Ethernet frame, so textbooks place it at Layer 2, 3, or “2.5”. TLS establishes a session (Layer 5) and encrypts data (Layer 6), but despite its name, it is not on the transport layer. For open-ended questions, your reasoning matters; for multiple-choice questions, use the classification taught in your course.

Yes. Two common mnemonics use words beginning with the same letters as the layers. For the English layer names from layer 1 to 7: “Please Do Not Throw Sausage Pizza Away” for Physical, Data Link, Network, Transport, Session, Presentation and Application. For the German names from layer 7 to 1: “Alle Deutschen Schüler Trinken Verschiedene Sorten Bier” for Application, Presentation, Session, Transport, Network, Data Link and Physical. Both mnemonics appear in practice mode and on the cheat sheet.

For each term, the corresponding layer in both models and an explanation. Search for ARP to see 2 Data link layer / 3 Network layer in the OSI model and Network access layer / Internet layer in the TCP/IP model, plus why textbooks classify it differently. Click a layer to see its function, data unit, addressing, devices and protocols.

Mainly for troubleshooting, working up through the layers from the bottom: Is the cable connected and is the network card's light on (1)? Can the computer see other devices on the local network, such as in the ARP table (2)? Does the gateway respond to ping (3)? Is the port open (4)? Is the service running (7)? Traceroute and the port checker help with Layers 3 and 4.

The tool is free and requires no sign-up. It runs entirely in your browser; your input is not sent to a server. Only your search term appears in the address bar so you can share a match, and your browser stores your quiz high scores locally.