OSI Layer Quiz – Name Each Layer and Its Data Unit

All 11 layers on the diagram

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OSI model: 7 layers 7

Physical layer (data unit: Bits) Turns bits into radio, electrical or optical signals. Example: Ethernet cabling.
Data link layer (data unit: Frame) Node to node over one link, with error detection. Example: Ethernet, Wi-Fi.
Network layer (data unit: Packet) Routing and relaying across networks, by address. Example: IP.
Transport layer (data unit: Segment) End to end between two hosts. Example: TCP, UDP.
Session layer (data unit: Data) Opens, keeps in step and closes a dialogue. Example: remote procedure calls.
Presentation layer (data unit: Data) Translates data into a shared format. Example: EBCDIC text to ASCII.
Application layer (data unit: Data) Where a program reaches the network. Example: HTTP, SMTP.

TCP/IP model: 4 layers 4

Link layer (TCP/IP) (data unit: Frame) Beside OSI layer 2; layer 1 is disputed. Example: PPP, Ethernet.
Internet layer (TCP/IP) (data unit: Packet) Beside OSI layer 3. Carries packets from network to network. Example: IP.
Transport layer (TCP/IP) (data unit: Segment) Beside OSI layer 4, and the same job. Example: TCP, UDP.
Application layer (TCP/IP) (data unit: Data) Beside OSI layers 7, 6 and 5. Example: HTTP, SMTP, DNS.

How this list was compiled: Names, numbers and order of the seven layers, and the job of each in our own words: ITU-T Recommendation X.200 (07/94), Open Systems Interconnection, Basic Reference Model, the identical text of ISO/IEC 7498-1:1994, clause 6.1.2 and clauses 7.1 to 7.7, read on 2026-09-30 in the PDF linked from https://www.itu.int/rec/T-REC-X.200-199407-I/en. X.200 never says segment, packet or frame; it speaks of an (N)-protocol-data-unit. The same seven layers from the bottom up, with raw bits at the physical layer, the frame at the data link layer and the packet at the network layer: L. Peterson and B. Davie, Computer Networks: A Systems Approach, section 1.3 (the OSI model), https://book.systemsapproach.org/foundation/architecture.html, read 2026-09-30. That textbook calls the transport unit a message, which is why message is neither asked nor accepted. The four TCP/IP layers (Application, Transport, Internet, Link), the sentence that the application layer combines the top two OSI layers, and the definitions of segment, IP datagram, packet and frame: RFC 1122, Requirements for Internet Hosts, Communication Layers, October 1989, sections 1.1.3 and 1.3.3, https://www.rfc-editor.org/rfc/rfc1122.txt, read 2026-09-30. The five everyday unit names by OSI layer (data, segment, packet, frame, bit), the example protocols of each layer and the remark that TLS fits no single layer: Wikipedia, Protocol data unit, https://en.wikipedia.org/wiki/Protocol_data_unit and OSI model, https://en.wikipedia.org/wiki/OSI_model, read 2026-09-30. Which OSI layers each TCP/IP layer is drawn beside, the other names books give the lowest layer (network access) and the note that authors disagree about layer 1: Wikipedia, Internet protocol suite, https://en.wikipedia.org/wiki/Internet_protocol_suite, read 2026-09-30. Hints, detail lines and the order of the questions are our own. Licence: own work

The drawing: Original drawing made for this site in 2026 from plain geometry. The layout, the pictures and the numbering are our own; nothing is traced or copied from a textbook, a course or a vendor poster. Licence: own work

How it works

One block of the stack lights up. Pick its name from four choices, then pick what data is called at that layer. The round climbs from layer 1 to layer 7, then moves to the four TCP/IP layers.

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How a round goes

A block lights up with its number beside it. Choose its name, then its data unit: bits, frame, packet, segment or data. The name is written into the block and stays there. After layer 7 the drawing changes to the TCP/IP model, where each of the four layers sits beside the OSI layers it covers. The round ends with the layers you missed.

The seven layers, bottom to top

1 Physical, 2 Data link, 3 Network, 4 Transport, 5 Session, 6 Presentation, 7 Application. Layers 7, 6 and 5 all carry plain data. A header is added at each of layers 4, 3 and 2, so the name changes there: segment, packet, frame. Layer 1 sends the frame as bits.

From layer 1 up, the first letters are P, D, N, T, S, P, A.
Two layers start with P. Physical is layer 1, the cable end. Presentation is layer 6, one below the top.
Segment, packet and frame are easy to swap. Count the header cells on the bar beside the block: one, two, three.

Questions and answers

What are the 7 layers of the OSI model in order?

From the top: 7 Application, 6 Presentation, 5 Session, 4 Transport, 3 Network, 2 Data link, 1 Physical. The names and numbers come from ITU-T Recommendation X.200, the same text as ISO/IEC 7498-1, which is free to read at itu.int.

What is the data unit at each OSI layer?

By convention: data at layers 7, 6 and 5, a segment at layer 4, a packet at layer 3, a frame at layer 2 and bits at layer 1. The standard only says protocol data unit. RFC 1122 defines segment, packet and frame for TCP/IP.

How do the TCP/IP layers match the OSI layers?

RFC 1122 names four: Application, Transport, Internet and Link. Transport matches layer 4 and Internet matches layer 3. Books draw Application beside layers 7, 6 and 5, though the RFC says it combines the top two. Whether Link reaches down to layer 1 is disputed.

Which protocols work at which OSI layer?

The usual examples: HTTP and SMTP at layer 7, TCP and UDP at layer 4, IP at layer 3, Ethernet and Wi-Fi at layer 2. Real protocols were not built to the model, so some, such as TLS, fit no single layer.