Fundamentals

Infrastructure

The Internet is made up of many networks from different Internet Service Providers. These providers connect a large number of networks and data centres. There are two main categories of providers:

Backbone providers

Responsible for the main international cable routes

Local providers

Responsible for providing Internet endpoints to users

• Each connected device is given a unique address called IPv4 or IPv6.

• Domains can be used as aliases for human readable representation.

  • These are translated to IPv4/IPv6 via DNS.

  • See DNS

OSI Model

The OSI model is a standard for describing the operation of connections between two devices. It consists of 7 different layers, each with different responsibilities and functions.

Image of OSI model with layers

1 - Physical Layer

Purpose: Transmits raw bits (0s and 1s) over the physical medium

Functions:

• Defines physical characteristics like cables and frequencies

• Converts data into signals and vice versa

• Examples: Ethernet cables, fiber optics, Wi-Fi (physical transmission)

2 - Data Link Layer

Purpose: Ensures reliable data transfer between directly connected devices

Functions:

• Handles error detection and correction

• Manages access to the physical medium

• Assigns physical addresses (MAC)

3 - Network Layer

Purpose: Handles the routing of data packets across different networks.

Functions:

• Assigns logical (IP) addresses.

• Determines the best path for data (routing).

• Handles packet fragmentation and reassembly.

• Examples: IP, ICMP, OSPF.

4 - Transport Layer

Purpose: Manages end-to-end communication and data transfer reliability.

Functions:

• Establishes, manages, and terminates connections.

• Ensures reliable data transfer with error correction and flow control.

• Breaks data into smaller segments and reassembles them at the destination.

• Examples: TCP (reliable), UDP (unreliable but faster).

5 - Session Layer

Purpose: Manages sessions or connections between applications.

Functions:

• Establishes, maintains, and terminates sessions.

• Synchronizes data exchanges (e.g., checkpoints for long transfers).

• Restores sessions after interruptions.

• Examples: SMB (file sharing), RPC (remote procedure calls).

6 - Presentation Layer

Purpose: Translates and formats data for the application layer.

Functions:

• Converts data formats (e.g., from ASCII to Unicode).

• Handles data encryption and compression.

• Examples: SSL/TLS, JPEG, MPEG.

IP Assignment

There are several ways to assign an IP address to clients. The most common method is dynamic assignment via DHCP. Another option is to assign static IP addresses to clients.

IP addresses are assigned to network interfacees. This is why you can see multiple ip addresses in ip a or ipconfig.

Private Networks

NAT

Resources

What is TCP 3 Way Handshake? - Scaler BlogScaler Blog