🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Technology & Design › IT Support / Networking › Networking Fundamentals and Infrastructure › Lesson 1
TEC-ITS43-04 · Lesson 1 · 10 clock hours · theory

Network Models: OSI and TCP/IP

Objective. Students will identify all seven OSI layers and the four TCP/IP model layers, name at least three protocols operating at each layer, and explain encapsulation and de-encapsulation with a written diagram.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Network Models Matter

When data travels from your browser to a web server thousands of miles away, dozens of hardware devices and software processes must cooperate perfectly. Network engineers use two reference models—the OSI model and the TCP/IP model—to organize that cooperation. Both models break communication into layers so that designers, technicians, and vendors can build products that work together regardless of brand or operating system. The CompTIA Network+ and Cisco CCNA exams test these models extensively, so mastering them now builds the foundation for every certification you will earn in this program.

The OSI Model: Seven Layers

The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO) and published as a conceptual framework for how network functions should be separated. It has seven layers, numbered 1 through 7 from the bottom up.

Layer 1 – Physical: Transmits raw bits over a medium—copper wire, fiber-optic cable, or radio waves. Protocols and standards here include Ethernet physical specifications (100BASE-T, 1000BASE-LX), USB, and Bluetooth radio. Devices: hubs, repeaters, cables.

Layer 2 – Data Link: Packages bits into frames and handles node-to-node delivery on the same network segment using MAC addresses. Protocols include Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11), and PPP. Devices: switches, wireless access points, NICs.

Layer 3 – Network: Routes packets between different networks using logical (IP) addresses. Protocols include IPv4, IPv6, ICMP, and OSPF. Devices: routers, Layer 3 switches.

Layer 4 – Transport: Provides end-to-end communication, error recovery, and flow control between hosts. TCP offers reliable, connection-oriented delivery; UDP offers faster, connectionless delivery. Other protocols: SCTP.

Layer 5 – Session: Establishes, manages, and terminates communication sessions between applications. Protocols include NetBIOS, RPC, and PPTP.

Layer 6 – Presentation: Translates data formats, handles encryption/decryption, and manages compression. Examples: SSL/TLS (encryption), JPEG, MPEG, ASCII, Unicode.

Layer 7 – Application: Closest to the end user; provides network services directly to applications. Protocols include HTTP, HTTPS, FTP, SMTP, DNS, Telnet, SSH, and SNMP.

A common memory aid for layers 1–7 is: Please Do Not Throw Sausage Pizza Away (Physical, Data Link, Network, Transport, Session, Presentation, Application).

The TCP/IP Model: Four Layers

The TCP/IP model (also called the Internet model or DoD model) describes how the protocols actually used on the modern Internet are organized. It maps closely to the OSI model but combines some layers.

| TCP/IP Layer | Corresponds to OSI Layers | Example Protocols |

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

| Network Access (Link) | 1 & 2 | Ethernet, Wi-Fi (802.11), ARP |

| Internet | 3 | IPv4, IPv6, ICMP, OSPF |

| Transport | 4 | TCP, UDP, SCTP |

| Application | 5, 6, & 7 | HTTP, HTTPS, FTP, DNS, SMTP, SSH, SNMP |

The TCP/IP model is the framework used by real implementations; OSI is the universal reference for troubleshooting discussions.

Encapsulation and De-encapsulation

Encapsulation is the process of adding layer-specific header (and sometimes trailer) information to data as it travels down the OSI stack on the sending device. Each layer wraps the data unit from the layer above.

```

Application → DATA (called: data or message)

Transport → TCP/UDP HEADER | DATA (called: segment [TCP] or datagram [UDP])

Network → IP HEADER | SEGMENT (called: packet)

Data Link → FRAME HEADER | PACKET | FRAME TRAILER (called: frame)

Physical → bits transmitted over the medium

```

De-encapsulation is the reverse: the receiving device reads and strips each header at the corresponding layer as data travels up the stack, until the original application data is delivered.

Data unit names to memorize: Data/Message → Segment → Packet → Frame → Bits.

Protocol Quick Reference

Safety and Professional Practice Notes

When working in wiring closets or data centers where physical-layer equipment is installed, always follow your facility's lockout/tagout (LOTO) procedures before servicing powered patch panels or UPS units. Wear appropriate PPE (safety glasses, ESD wrist strap) when handling network interface cards or server components. Never open a powered UPS or high-voltage switch gear without proper electrical safety training. These physical-layer precautions protect you even though the conceptual models discussed today are software-focused.

Key terms10

OSI ModelA seven-layer conceptual framework developed by ISO that describes how data communication functions should be separated and standardized across different systems.
TCP/IP ModelA four-layer practical framework (Network Access, Internet, Transport, Application) that describes the protocols used on the modern Internet.
EncapsulationThe process of adding a layer-specific header (and sometimes trailer) to data as it passes down the OSI stack on the sending device.
De-encapsulationThe reverse of encapsulation; the receiving device strips each header at the corresponding OSI layer as data moves up the stack.
Protocol Data Unit (PDU)The name for the data unit at each OSI layer: data/message (Layer 7–5), segment or datagram (Layer 4), packet (Layer 3), frame (Layer 2), bits (Layer 1).
MAC AddressA hardware address burned into a network interface card, used at the Data Link layer (Layer 2) to identify devices on the same network segment.
IP AddressA logical address assigned to a device at the Network layer (Layer 3) that enables routing across different networks.
TCP (Transmission Control Protocol)A connection-oriented, reliable Transport layer protocol that guarantees delivery and correct ordering of data segments.
UDP (User Datagram Protocol)A connectionless, low-overhead Transport layer protocol used when speed is preferred over guaranteed delivery, such as in video streaming or DNS queries.
ARP (Address Resolution Protocol)A protocol used to resolve a known IP address to the corresponding MAC address on a local network segment, operating at the boundary of OSI Layers 2 and 3.
In class: Lecture and guided note-taking using an OSI reference chart; students label a blank model from memory and compare results with a partner · Protocol-sorting card activity: each card names a protocol (HTTP, TCP, IP, Ethernet, DNS, FTP, etc.); students place each card on the correct OSI and TCP/IP layer on a poster-sized model · Written scenario exercise: given a browser request to a web server, students trace the data unit name, header additions, and responsible layer at each step from application to physical and back
Materials: OSI/TCP-IP reference posters, CompTIA Network+ Study Guide (current edition), whiteboard, protocol sorting card sets (instructor-produced)
Check: Five-question oral quiz at lesson close; student must correctly place at least four of five protocols AND name the correct layer functions; score of 80% or higher required

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

CompTIA Network+ – Networking Concepts
1. A network technician needs to troubleshoot a routing problem between two different networks. Which OSI layer is MOST directly responsible for this function?
CompTIA Network+ – Networking Concepts
2. Which of the following BEST describes the process of encapsulation?
Cisco CCST – Networking Concepts
3. A user opens a browser and types a URL. DNS is used to resolve the domain name to an IP address. At which TCP/IP model layer does DNS operate?
CompTIA Network+ – Networking Concepts
4. Which data unit name is assigned to data at OSI Layer 3?
CompTIA A+ – Networking
5. An engineer is designing a system that must guarantee ordered, error-checked delivery of data between two hosts. Which Layer 4 protocol should be selected?
CompTIA Network+ – Networking Concepts
6. In the OSI model, at which layer does TLS encryption and decryption PRIMARILY occur?
Cisco CCNA – Network Fundamentals
7. A technician captures network traffic and sees a frame with a source MAC address, destination MAC address, and an embedded IP packet. At which OSI layer was this PDU formed?
CompTIA Network+ – Networking Concepts
8. When a web server receives an HTTP request from a client, it processes data traveling UP the OSI stack. Which sequence correctly describes de-encapsulation from Layer 2 to Layer 7?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
Program planNext lesson →