← OSI & TCP/IP Models – CompTIA Network+ Flashcards

CompTIA Network+ Certification Study Guide

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OSI & TCP/IP Models – CompTIA Network+ Study Guide


Overview

The OSI (Open Systems Interconnection) and TCP/IP models are foundational frameworks for understanding how data travels across networks. The OSI model provides a 7-layer theoretical reference, while the TCP/IP model is the practical 4-layer framework used in real-world networking. Mastering these models — including their layers, PDUs, protocols, and devices — is essential for the CompTIA Network+ exam.


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OSI Model Layers & Functions


The Seven Layers at a Glance


> Mnemonic (Layer 1→7): "Please Do Not Throw Sausage Pizza Away"

> Mnemonic (Layer 7→1): "All People Seem To Need Data Processing"


| Layer | Name | PDU | Key Function |

|-------|------|-----|-------------|

| 7 | Application | Data | User-facing network services |

| 6 | Presentation | Data | Translation, encryption, compression |

| 5 | Session | Data | Session setup, management, teardown |

| 4 | Transport | Segment / Datagram | End-to-end delivery, segmentation |

| 3 | Network | Packet | Logical addressing, routing |

| 2 | Data Link | Frame | MAC addressing, error detection |

| 1 | Physical | Bits | Raw bit transmission |


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Layer 1 – Physical

  • • Transmits raw binary bits (0s and 1s) over a physical medium
  • • Defines electrical signals, voltage levels, cable types, and physical connectors
  • • Devices: Hubs, repeaters, cables, NICs (physical component)

    • Layer 2 – Data Link

  • • Divided into two sublayers:

    • - LLC (Logical Link Control): Interfaces with Layer 3

    - MAC (Media Access Control): Handles hardware addressing

  • • Uses MAC addresses for physical addressing within a local network
  • • Adds a CRC trailer for error detection
  • • Devices: Switches (Layer 2), bridges
  • • PDU: Frame

    • Layer 3 – Network

  • • Handles logical (IP) addressing and routing between different networks
  • • Determines the best path for packet delivery
  • • Protocols: IP, ICMP, ARP (boundary L2/L3)
  • • Devices: Routers, Layer 3 switches
  • • PDU: Packet

    • Layer 4 – Transport

  • • Provides end-to-end communication and segmentation
  • • Two main protocols:

    • - TCP – Connection-oriented, reliable, ordered delivery, acknowledgments

    - UDP – Connectionless, faster, unreliable delivery

  • • Manages flow control and error recovery (TCP)
  • • PDU: Segment (TCP) / Datagram (UDP)

    • Layer 5 – Session

  • • Establishes, manages, and terminates sessions between applications
  • • Handles session checkpointing and recovery
  • • Examples: NetBIOS, RPC, SQL sessions

    • Layer 6 – Presentation

  • • Translates data between application format and network format
  • • Handles encryption/decryption (TLS/SSL) and compression
  • • Ensures data is in a usable format for the Application layer
  • • Examples: TLS/SSL, JPEG, ASCII, MPEG

    • Layer 7 – Application

  • • Provides network services directly to end-user applications
  • • Closest layer to the end user
  • • Protocols: HTTP, FTP, DNS, SMTP, DHCP, SNMP

    • ---


    Key Terms – OSI Model

  • • LLC (Logical Link Control): Upper sublayer of Data Link; interfaces with Network layer
  • • MAC (Media Access Control): Lower sublayer of Data Link; handles hardware addressing
  • • CRC (Cyclic Redundancy Check): Error detection value added as a trailer at Layer 2
  • • Logical Address: IP address used at Layer 3 for routing between networks
  • • Physical Address: MAC address used at Layer 2 for local delivery

    • ---


    Watch Out For – OSI Layers

    > ⚠️ Layer 2 vs. Layer 3 Switches: A standard switch = Layer 2 (MAC addresses only). A Layer 3 switch can ALSO route using IP addresses — it combines switch + router functionality.


    > ⚠️ ARP lives at the boundary: ARP is often listed as Layer 2 or Layer 3 — it resolves IP addresses (L3) to MAC addresses (L2). On the exam, expect either answer depending on context.


    > ⚠️ TLS/SSL = Layer 6, NOT Layer 7: Students frequently place encryption at the Application layer. It belongs at the Presentation layer.


    > ⚠️ Routers use IP addresses; Switches use MAC addresses. Don't mix these up on device/layer questions.


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    Encapsulation & Protocol Data Units (PDUs)


    Encapsulation Process (Sending – Top to Bottom)

    Each layer adds a header (and Layer 2 adds a trailer) to the data before passing it down:


    ```

    Application Data

    ↓ [+ L7/L6/L5 headers]

    Transport Layer → Segment (TCP) or Datagram (UDP)

    ↓ [+ L4 header]

    Network Layer → Packet

    ↓ [+ L3 header]

    Data Link Layer → Frame [+ L3 header + CRC trailer]

    ↓ [+ L2 header + trailer]

    Physical Layer → Bits (0s and 1s)

    ```


    De-encapsulation Process (Receiving – Bottom to Top)

  • • Each layer on the receiving device strips off its corresponding header/trailer
  • • Passes the remaining data up to the next layer
  • • Reverses the encapsulation process completely

    • PDU Quick Reference


    | OSI Layer | PDU Name | Contains |

    |-----------|----------|---------|

    | Layer 4 (Transport) | Segment (TCP) / Datagram (UDP) | Source/dest ports, sequence numbers |

    | Layer 3 (Network) | Packet | Source/dest IP addresses |

    | Layer 2 (Data Link) | Frame | Source/dest MAC addresses + CRC trailer |

    | Layer 1 (Physical) | Bits | Raw 0s and 1s |


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    Key Terms – Encapsulation

  • • Encapsulation: Adding headers/trailers to data moving down the OSI stack
  • • De-encapsulation: Removing headers/trailers from data moving up the OSI stack
  • • PDU (Protocol Data Unit): The name for data at each specific OSI layer
  • • Trailer: Only added at Layer 2; contains the CRC for error detection
  • • Header: Added at Layers 2–4 containing control information

    • ---


    Watch Out For – Encapsulation

    > ⚠️ Only Layer 2 adds a TRAILER. All other layers add only headers. The CRC trailer is unique to the Data Link layer.


    > ⚠️ PDU names matter on the exam. Know: Bits (L1) → Frame (L2) → Packet (L3) → Segment/Datagram (L4). Mixing these up is a common mistake.


    > ⚠️ TCP = Segment | UDP = Datagram. Both are Layer 4, but use different PDU names.


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    TCP/IP Model


    Four Layers & OSI Mapping


    | TCP/IP Layer | OSI Equivalent | Key Protocols |

    |-------------|---------------|--------------|

    | Application | OSI Layers 5, 6, 7 | HTTP, FTP, DNS, SMTP, DHCP |

    | Transport | OSI Layer 4 | TCP, UDP |

    | Internet | OSI Layer 3 | IP, ICMP, ARP |

    | Network Access (Link) | OSI Layers 1 & 2 | Ethernet, Wi-Fi |


    Layer Descriptions


    #### Network Access Layer (Link Layer)

  • • Maps to OSI Layers 1 & 2 (Physical + Data Link)
  • • Handles physical transmission and MAC addressing
  • • Protocols: Ethernet, Wi-Fi (802.11)

    • #### Internet Layer

  • • Maps to OSI Layer 3 (Network)
  • • IP (Internet Protocol) is the primary protocol
  • • Handles logical addressing and packet routing
  • • Also includes ICMP and ARP

    • #### Transport Layer

  • • Maps to OSI Layer 4
  • • TCP (reliable) and UDP (fast/unreliable)
  • • End-to-end communication between hosts

    • #### Application Layer

  • • Maps to OSI Layers 5, 6, AND 7 (Session + Presentation + Application)
  • • All upper-layer protocols live here in TCP/IP

    • ---


    Key Terms – TCP/IP

  • • IP (Internet Protocol): Core protocol at the Internet layer; handles logical addressing and routing
  • • TCP (Transmission Control Protocol): Reliable, connection-oriented Transport layer protocol
  • • UDP (User Datagram Protocol): Fast, connectionless Transport layer protocol
  • • Network Access Layer: TCP/IP's combined physical + data link layer

    • ---


    Watch Out For – TCP/IP Model

    > ⚠️ TCP/IP Application layer ≠ OSI Application layer. The TCP/IP Application layer includes Session (L5), Presentation (L6), AND Application (L7) from the OSI model.


    > ⚠️ TCP/IP has 4 layers; OSI has 7. Don't mix up layer numbers between the two models on the same question.


    > ⚠️ The bottom TCP/IP layer has multiple names: Network Access, Link, or Network Interface layer — all refer to the same layer.


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    Protocols & Layer Mapping


    Protocol Reference Chart


    | Protocol | OSI Layer | TCP/IP Layer | Port(s) | Notes |

    |----------|-----------|-------------|---------|-------|

    | HTTP | L7 Application | Application | TCP 80 | Web traffic |

    | HTTPS | L7 Application | Application | TCP 443 | Secure web (uses TLS) |

    | DNS | L7 Application | Application | UDP/TCP 53 | UDP for queries; TCP for zone transfers |

    | FTP | L7 Application | Application | TCP 20/21 | File transfers |

    | SMTP | L7 Application | Application | TCP 25 | Email sending |

    | TLS/SSL | L6 Presentation | Application | — | Encryption/decryption |

    | TCP | L4 Transport | Transport | — | Reliable, connection-oriented |

    | UDP | L4 Transport | Transport | — | Fast, connectionless |

    | IP | L3 Network | Internet | — | Logical addressing + routing |

    | ICMP | L3 Network | Internet | — | Ping, traceroute, error reporting |

    | ARP | L2/L3 boundary | Internet/Network Access | — | IP → MAC resolution |

    | Ethernet | L2 Data Link | Network Access | — | MAC addressing + framing |


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    TCP vs. UDP Comparison


    | Feature | TCP | UDP |

    |---------|-----|-----|

    | Connection type | Connection-oriented | Connectionless |

    | Reliability | Reliable (acknowledgments) | Unreliable (no ACKs) |

    | Speed | Slower | Faster |

    | Order | Ordered delivery | No guaranteed order |

    | Use cases | HTTP, FTP, email | DNS, VoIP, video streaming |

    | PDU name | Segment | Datagram |


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    Network Devices by OSI Layer


    | Device | OSI Layer | Addressing Used |

    |--------|-----------|----------------|

    | Hub / Repeater | Layer 1 | None (broadcasts all) |

    | Switch (L2) | Layer 2 | MAC addresses |

    | Switch (L3) | Layers 2–3 | MAC + IP addresses |

    | Router | Layer 3 | IP addresses |

    | Firewall (NGFW) | Layers 3–7 | IP, ports, application data |

    | Proxy / Load Balancer | Layer 7 | Application content |


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    Key Terms – Protocols

  • • ARP (Address Resolution Protocol): Resolves IP addresses to MAC addresses at the L2/L3 boundary
  • • ICMP (Internet Control Message Protocol): Layer 3 protocol used for diagnostics (ping, traceroute)
  • • DNS (Domain Name System): Resolves hostnames to IP addresses; uses UDP 53 / TCP 53
  • • NGFW (Next-Generation Firewall): Can inspect traffic across all 7 OSI layers

    • ---


    Watch Out For – Protocols

    > ⚠️ DNS uses BOTH UDP and TCP port 53. UDP for standard queries; TCP for zone transfers and large responses. Know BOTH.


    > ⚠️ ICMP has no port number. It operates at Layer 3 and is not TCP or UDP — it won't have a port number on the exam.


    > ⚠️ HTTP = port 80 (not encrypted); HTTPS = port 443 (encrypted with TLS/SSL). Don't confuse the two.


    > ⚠️ Ping uses ICMP, not TCP/UDP. It's a Layer 3 tool, not a Transport layer tool.


    > ⚠️ A hub is NOT a switch. Hubs operate at Layer 1 (broadcast all data); switches operate at Layer 2 (forward based on MAC tables).


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    Quick Review Checklist


    Before your exam, make sure you can confidently answer each of the following:


    OSI Model

  • • [ ] Recite all 7 OSI layers in order (both top-to-bottom and bottom-to-top)
  • • [ ] State the primary function of each OSI layer
  • • [ ] Identify which layer each major network device operates at
  • • [ ] Explain the difference between a Layer 2 switch and a Layer 3 switch
  • • [ ] Identify the two sublayers of the Data Link layer (LLC and MAC)

    • Encapsulation & PDUs

  • • [ ] Name the PDU for each OSI layer (Bits, Frame, Packet, Segment/Datagram, Data)
  • • [ ] Explain encapsulation (top→bottom, adding headers/trailers)
  • • [ ] Explain de-encapsulation (bottom→top, stripping headers/trailers)
  • • [ ] Identify that ONLY Layer 2 adds a trailer (CRC)
  • • [ ] Distinguish between a TCP Segment and a UDP Datagram

    • TCP/IP Model

  • • [ ] Name the 4 TCP/IP layers in order
  • • [ ] Map each TCP/IP layer to its corresponding OSI layer(s)
  • • [ ] Explain that the TCP/IP Application layer = OSI Layers 5+6+7
  • • [ ] Explain that the Network Access layer = OSI Layers 1+2

    • Protocols & Layer Mapping

  • • [ ] Place HTTP, DNS, FTP, SMTP at Layer 7; know their port numbers
  • • [ ] Place TLS/SSL at Layer 6 (Presentation)
  • • [ ] Place TCP and UDP at Layer 4; explain key differences
  • • [ ] Place IP and ICMP at Layer 3
  • • [ ] Explain ARP's position at the Layer 2/3 boundary
  • • [ ] Know DNS uses UDP port 53 (queries) AND TCP port 53 (zone transfers)
  • • [ ] Know ICMP has no port number and is used by ping/traceroute

    • ---


    Study Tip: When answering OSI questions on the exam, use the mnemonic to quickly orient yourself, then work from the device or protocol to its layer. Practice mapping protocols, devices, AND PDUs together for each layer.

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