Layer 7 – Application Layer

• Where users interact with the computer
• Acts as an interface between an application and end-user protocols
• Provides an interface to communicate with the network (Outlook, Chrome, etc.)
• Applications don't reside in the application layer but instead interface with application-layer protocols
• Example Protocols:
  • E-Mail: IMAP4, POP3, SMTP
  • Web Browsers: HTTP, HTTPS
  • Remote Access: SSH, Telnet
• Real-World Examples:
  • Opening https://google.com in Chrome → uses HTTPS (port 443)
  • Sending an email via Gmail → uses SMTP (port 587) to send, IMAP (port 993) to receive
  • Connecting to a remote server via PuTTY → uses SSH (port 22)
  • Transferring files to a web server → uses FTP (port 21) or SFTP (port 22)
  • Resolving "google.com" to an IP address → uses DNS (port 53)

Layer 6 – Presentation Layer

• Ensures data transferred from one system's Application Layer can be read by the Application Layer on another
• Provides character code conversion, data compression, and data encryption/decryption
• Real-World Examples:
  • Visiting a website over HTTPS → TLS/SSL encrypts data at the Presentation Layer before sending
  • Uploading a JPEG photo to Instagram → image is compressed/formatted so any device can display it
  • Streaming a video on YouTube → video is encoded in MPEG/H.264 and decoded by your browser
  • Opening a Word document on Mac that was created on Windows → character encoding (e.g. UTF-8) ensures text displays correctly
  • Compressing a file into a .zip before emailing → data compression handled at this layer

Layer 6 – File Formats

• Web Browser: HTML, XML, JavaScript
• Graphics Files: JPEG, GIF, PNG
• Audio/Video: MPEG, MP3
• Encryption: TLS, SSL
• Text/Data: ASCII, EBCDIC

Layer 5 – Session Layer

• Responsible for setting up, managing, and tearing down sessions between network devices
• Ensures data from different application sessions are kept separate
• Utilises Application Program Interfaces (APIs) to communicate with TCP/IP protocols
• Coordinates communication between systems
  • Start, Stop, Restart
• Real-World Examples:
  • Logging into Netflix and streaming a movie → the Session Layer keeps your stream active and separate from other users
  • A video call on Zoom → the session is established, maintained throughout the call, and torn down when you hang up
  • Opening multiple tabs in Chrome → each tab maintains its own separate session with different websites
  • Resuming a failed file download → the session layer can restart the transfer from where it left off
  • Using NetBIOS to discover shared printers on a local network → session is created between your PC and the printer

Communication Modes

• Simplex: One-way communication between two devices, like listening to a radio station
  • e.g. TV broadcast, FM radio, keyboard sending input to computer
• Half Duplex: Two-way communication, but only one device can communicate at a time
  • e.g. Walkie-talkies (push-to-talk), CB radio, older Wi-Fi standards
• Full Duplex: Two-way communication where both sides can communicate at the same time
  • e.g. Phone call (both parties talk simultaneously), Ethernet over twisted-pair cables, video call on Zoom

Layer 4 – Transport Layer

• Ensures data is delivered error-free and in sequence
• Segments data and reassembles correctly
• Can be connection-oriented or connectionless
• Considered the "Post Office" Layer
  • TCP (Transmission Control Protocol)
  • UDP (User Datagram Protocol)
• Real-World Examples:
  • Loading a webpage → TCP ensures all HTML, CSS, and images arrive complete and in order
  • Sending an email → TCP guarantees the message is delivered reliably without missing data
  • Streaming a live game on Twitch → UDP is used for speed; a few lost frames are acceptable
  • Online gaming (e.g. Fortnite) → UDP sends rapid position updates; low latency matters more than perfection
  • Voice over IP (VoIP) phone call → UDP keeps the conversation in real-time without waiting for retransmissions

Layer 4 – Flow Control

Responsible for two data flow control measures:

• Buffering
  • Stores data in memory buffers until destination device is available
• Windowing
  • Allows devices in session to determine the "window" size of data segments sent
• Real-World Examples:
  • Downloading a large file → Buffering stores incoming chunks in memory while the disk writes catch up
  • Streaming a YouTube video → the progress bar loads ahead using buffering so playback stays smooth
  • A fast server sending to a slow IoT device → Windowing shrinks the segment size so the device isn't overwhelmed
  • TCP "slow start" on a new connection → the window begins small and grows as the network proves it can handle more
  • A printer receiving a large document → Buffering queues pages in memory so the sender doesn't have to wait for each page to print

Layer 3 – Network Layer

• The "Routing" Layer
• Provides logical addressing (IP Addressing) and routing services
• Places two IP addresses into a packet:
  • Source Address & Destination IP Address
• Internet Protocol (IP)
  • The primary network protocol used on the Internet
  • IPv4, IPv6 Logical Addresses
• Real-World Examples:
  • Your laptop gets an IP address (e.g. 192.168.1.10) from the router so it can be identified on the network
  • A home router decides the best path to forward your Google search from your LAN to the Internet
  • Typing a URL → DNS returns an IP address, and the Network Layer uses it to route your request across multiple networks

Layer 3 – Packets & Devices

Layer 2 – Data Link Layer

• The "Switching" Layer
• Ensures messages are delivered to the proper device on a LAN using hardware addresses
  • MAC (Media Access Control) Address
  • Only concerned with local delivery of frames on the same network
• Responsible for packaging data into frames for the Physical Layer
• Translates messages from the Network Layer into bits for the Physical Layer
• Real-World Examples:
  • Your laptop's MAC address (e.g. AA:BB:CC:DD:EE:FF) uniquely identifies its network card on the local network
  • A network switch reads MAC addresses to forward frames only to the correct port, not the entire network
  • An Ethernet frame wraps your IP packet with source and destination MAC addresses before it hits the cable

Layer 2 – Sub-Layers

• Real-World Examples:
  • LLC: When a Wi-Fi frame arrives corrupted, the LLC sub-layer detects the error and requests retransmission
  • LLC: On a busy network, LLC flow control slows down a fast server so a slower printer's buffer doesn't overflow
  • MAC: Every network card has a unique 48-bit address (e.g. 00:1A:2B:3C:4D:5E) burned in at the factory
  • MAC: Wired Ethernet uses CSMA/CD — if two devices transmit at once, they detect the collision and retry after a random delay
  • MAC: Wi-Fi uses CSMA/CA — devices listen first and wait for a clear channel before transmitting to avoid collisions

Layer 1 – Physical Layer

• Defines the physical and electrical medium for network communication:
  • Sending and receiving bits (1 or 0)
  • Encoding Signal Types – Electricity, radio waves, light
  • Network Cabling, Jacks, Patch Panels – Copper or Fiber
  • Physical Network Topology – Star, Mesh, Ring, etc.
  • Ethernet IEEE 802.3 Standard
• Layer 1 Equipment: Hubs, Media Converters, Modems
• Responsible for the network hardware and physical topology

OSI Model Summary

Data Encapsulation

As data moves down the OSI model, each layer adds its own header (and sometimes trailer):

Physical vs Logical Flow

Data travels down the OSI layers on the sending device, across the physical medium, through intermediary devices (switches at Layer 2, routers at Layer 3), and then up the layers on the receiving device.

Understanding IPv4 Addresses

• An IP Address is a logical address used to uniquely identify a device on an IP network
• It's a Network Layer address (Layer 3 of the OSI Model)
• There are two versions:
  • IP version 4 (IPv4)
  • IP version 6 (IPv6)
• This lesson focuses on IPv4 — we'll discuss IPv6 later in the course

IPv4 Address Anatomy

• Made up of 32 binary bits, divided into a network portion and a host portion with the help of a subnet mask
• The 32 binary bits are broken into four octets (1 octet = 8 bits)
• Each octet is converted to decimal and separated by a period (dot)
• An IP address is expressed in dotted decimal format

IPv4 Address Breakdown

Breaking down the address 192.168.1.131 into its four octets:

Network & Host Portion

An IP address is broken into two parts:

Network Address + Host Address = IP Address

IPv4 Address Components

Each device on a network is assigned:

• IP Address: Unique logical address assigned to each device on a network
• Subnet Mask: Used by the device to determine what subnet it's on — specifically the network and host portions of the IP address
• Default Gateway: The IP address of a network's router that allows devices on the local network to communicate with other networks

Basics of Binary Math

Binary → Decimal: 11111111

Add the value where there is a "1". Add zero when there is a "0".

Binary → Decimal: 10101010

Add the value where there is a "1". Add zero when there is a "0".

Binary → Decimal: 10000011

Add the value where there is a "1". Add zero when there is a "0".

Decimal → Binary: 192

Divide by 2 repeatedly. The remainders (read bottom → top) give the binary number:

Read remainders ↑ bottom to top: 11000000

Decimal → Binary: 202

Start adding the values from left to right until you reach the target decimal!

Decimal → Binary: 54

Start adding the values from left to right until you reach the target decimal!

IP Address Conversion Process

Whether given an IP in dotted-decimal or binary, convert each octet one by one:

192.168.32.4 = 11000000.10101000.00100000.00000100