Name: The Foundation of Iron — Week 17 — OSI Model and Advanced Addressing
Author: Amine RAITI

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THE FOUNDATION OF IRON · COURSE MATERIAL · WEEK 17

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OSI MODEL AND
ADVANCED ADDRESSING
Week 17 of 26 · Block 6 — Foundational Networking
12h theory · 23h practice

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◆ WEEKLY LEARNING OBJECTIVES

1. Describe the 7 OSI layers and the role of each
2. Explain the encapsulation and decapsulation mechanism
3. Master subnet calculation (network address, broadcast, host range)
4. Capture and analyse frames with a network analyser
5. Identify the layer involved in a common network problem

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⚠ WARNING — SHELF LIFE OF VERSIONS REFERENCED HERE

The OSI model is a stable standard. Network analysis tools (Wireshark and its alternatives) however evolve regularly. The instructor verifies the menus and options actually available in the version installed in the classroom at the time of delivery.

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Amine RAITI · Infrastructure Architect & SRE
Public document · CC BY-NC-SA 4.0 · AI Powered by Amine
Opération Dindon

RATIO

COURSE OUTLINE · 12H

THEORY GUIDING THREAD

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17.1 · The 7 OSI layers and their roles4h

— Physical (1): cable, electrical signal (direct link with Week 1 — voltage and current)
— Data Link (2): MAC addressing, media access, Ethernet frame
— Network (3): IP addressing, routing between networks (link with Week 2 — IP addresses in binary)
— Transport (4): segmentation, TCP (reliable) vs UDP (fast), ports
— Session (5), Presentation (6), Application (7): conceptual introduction

17.2 · Encapsulation and decapsulation3h

— Each layer adds a header by encapsulating the data from the layer above
— Tracing the path of an HTTP packet from the application down to the physical cable
— PDU (Protocol Data Unit): bit → frame → packet → segment → data

17.3 · Advanced subnet calculation3h

— Recap of CIDR notation (direct link with Week 2 — masks in binary)
— Calculating the network address (logical AND), broadcast, host range
— Subnetting exercises: dividing a network into different-sized subnets
— Address conflict resolution: detecting and fixing a duplicate address

17.4 · Network diagnosis by layer2h

— Bottom-up diagnosis method: start at the physical layer (cable, connector) before moving up to higher layers
— Basic diagnostic tools and their target layer: ping (layer 3), traceroute (layer 3), netstat/ss (layer 4)

RATIO

EXERCISE 1 · SUBNET CALCULATION AND FRAME CAPTURE · 12H

Equipment: provided exercise sheet (10 subnet calculation cases), Wireshark installed on the classroom machines.

(3h) Manual calculation for 10 IP addresses with various CIDR masks: network address, broadcast address, usable host range, number of possible hosts — no tools, only the binary method from Week 2.

(2h) Subnetting exercise: divide the 192.168.10.0/24 network into 4 equal subnets, then into different-sized subnets (30 hosts, 14 hosts, 6 hosts).

(4h) Frame capture with Wireshark: start a capture, generate different types of traffic (ping, HTTP connection, SSH connection), analyse the captured frames and identify the OSI layers present in each frame.

(3h) Simulated network fault diagnosis: the instructor introduces 3 network configuration problems on the VMs from previous weeks (wrong IP address, wrong mask, wrong gateway) — the trainee diagnoses each problem by layer and fixes it.

SOLUTION — EXERCISE 1

Example subnetting (/24 divided into 4 /26 subnets):
Subnet 1: 192.168.10.0/26 (hosts .1 to .62, broadcast .63)
Subnet 2: 192.168.10.64/26 (hosts .65 to .126, broadcast .127)
Subnet 3: 192.168.10.128/26 (hosts .129 to .190, broadcast .191)
Subnet 4: 192.168.10.192/26 (hosts .193 to .254, broadcast .255)

Expected layer-by-layer diagnosis method: layer 1 (physical cable/connector), layer 2 (ARP table, MAC address), layer 3 (ping, IP address, mask, gateway), layer 4 (netstat/ss, port open or not).

RATIO

EXERCISE 2 · IN-DEPTH OSI FRAME ANALYSIS · 11H

Equipment: Wireshark, the Linux and Windows machines from previous weeks, network simulator (Cisco Packet Tracer or GNS3 as available).

(3h) Detailed analysis of a captured HTTP frame: identify each layer (Ethernet frame, IP packet, TCP segment, HTTP data), read the values of key fields (MAC addresses, source/destination IP, source/destination port).

(3h) Comparing a TCP frame and a UDP frame: observe the structural difference (no acknowledgement in UDP), analyse a DNS exchange (which uses UDP) and an SSH exchange (which uses TCP).

(3h) Building a simple topology in the simulator (2 machines, 1 switch) and simulating the communication: observe the ARP exchange, then the IP exchange, identify the PDUs at each step.

(2h) Writing an annotated diagram showing the complete path of an HTTP packet between two machines in the topology, layer by layer.

SOLUTION — EXERCISE 2

Expected diagram: the path of an HTTP packet should show the 4 layers active in practice (HTTP Application → TCP Transport → IP Network → Ethernet Data Link), with the corresponding addresses and ports at each layer, and the transformation into a physical signal at the cable level.

◆ SUMMARY SHEET — WEEK 17 SELF-ASSESSMENT

1. I can describe the role of each of the 7 OSI layers.

2. I can explain the encapsulation and decapsulation mechanism.

3. I can calculate the network address, broadcast and host range for a given CIDR.

4. I can divide a network into different-sized subnets.

5. I can capture and analyse frames with Wireshark.

6. I can identify OSI layers in a captured frame.

7. I can distinguish TCP and UDP and their typical use cases.

8. I can diagnose a common network problem using a layer-by-layer approach.