2.1 OSI Reference Model: Functions of Each Layer
The OSI (Open Systems Interconnection) Reference Model is a conceptual framework used to understand network interactions in seven distinct layers. Each layer has specific functions that help in data transmission and network communication.
- Physical Layer (Layer 1)
- Function: Responsible for the physical transmission of data over a medium (cables, wireless). It defines electrical signals, cable types, and wireless communication standards.
- Data Link Layer (Layer 2)
- Function: Handles error detection and correction, as well as the framing of data for transmission. It ensures that data is error-free as it travels across the physical layer.
- Network Layer (Layer 3)
- Function: Responsible for logical addressing and routing. It determines the best path for data to travel across the network (e.g., IP addressing, routing protocols).
- Transport Layer (Layer 4)
- Function: Ensures reliable data transfer by managing flow control, error recovery, and data segmentation (e.g., TCP, UDP protocols).
- Session Layer (Layer 5)
- Function: Manages sessions or connections between applications. It ensures that communication is orderly, handles synchronization, and manages dialogues between devices.
- Presentation Layer (Layer 6)
- Function: Translates data between the application layer and lower layers. It handles data compression, encryption, and translation into a format that applications can understand.
- Application Layer (Layer 7)
- Function: Provides network services to end-users, such as email, file transfer, and web browsing (e.g., HTTP, FTP, SMTP).
2.2 TCP/IP Reference Model
The TCP/IP (Transmission Control Protocol/Internet Protocol) Reference Model is the protocol suite that underpins the internet and most modern networks. It has four layers:
- Link Layer
- Function: Combines the OSI’s physical and data link layers. It is responsible for network access, including protocols like Ethernet, Wi-Fi, and ARP.
- Internet Layer
- Function: Corresponds to the OSI network layer. It handles logical addressing and routing (e.g., IP, ICMP).
- Transport Layer
- Function: Matches the OSI transport layer. It ensures reliable data transmission through protocols like TCP and UDP.
- Application Layer
- Function: Corresponds to the OSI session, presentation, and application layers. It provides services such as HTTP, FTP, and DNS.
Comparison of OSI and TCP/IP Reference Models
| Feature | OSI Model | TCP/IP Model |
| Layers | 7 (Physical, Data Link, Network, Transport, Session, Presentation, Application) | 4 (Link, Internet, Transport, Application) |
| Focus | Conceptual model for communication | Practical, real-world protocol suite |
| Use | More theoretical and detailed | Used in real-world networking (Internet) |
| Layer Functions | Detailed separation of functions | Combines some layers (e.g., Application layer covers OSI’s 5-7) |
2.3 TCP/IP Protocol Suite
The TCP/IP Protocol Suite is a set of protocols that enable the Internet to function. The core protocols in the suite include:
- TCP (Transmission Control Protocol): Provides reliable, connection-oriented communication. It ensures data arrives in the correct order and retransmits lost data.
- UDP (User Datagram Protocol): Provides fast, connectionless communication, suitable for applications where speed is more critical than reliability (e.g., live video streaming).
- IP (Internet Protocol): Handles addressing and routing of packets across networks (e.g., IPv4, IPv6).
- ARP (Address Resolution Protocol): Resolves IP addresses to MAC (Media Access Control) addresses.
- ICMP (Internet Control Message Protocol): Used for error reporting and diagnostics (e.g., ping command).
- HTTP/HTTPS: Protocols for transferring web pages over the internet.
2.4 Addressing
Addressing is crucial for identifying devices on a network. It involves several types of addresses:
2.4.1 Physical Addresses
- Also known as MAC (Media Access Control) addresses, these are unique identifiers assigned to network interfaces at the hardware level. They are used in the data link layer to facilitate communication between devices on the same network.
2.4.2 Logical Addresses
- IP addresses are logical addresses that uniquely identify devices on a network. They are used at the network layer to route data across different networks. There are two types:
- IPv4: 32-bit addresses (e.g., 192.168.1.1).
- IPv6: 128-bit addresses (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).
2.4.3 Port Addresses
- Port addresses are used by transport layer protocols (TCP/UDP) to specify the destination service or application on a device. Port numbers range from 0 to 65535, with well-known ports (0-1023) assigned to specific services (e.g., HTTP on port 80, HTTPS on port 443).
2.4.4 Specific Addresses
- Specific addresses can refer to broadcast addresses, which send data to all devices on a network, or multicast addresses, which send data to a selected group of devices.
2.5 IP Addressing
IP addressing assigns a unique identifier to every device on a network. It helps in routing packets between networks and ensures proper communication between devices.
2.5.1 Classful Addressing
- Classful addressing divides IP addresses into five classes: A, B, C, D, and E.
- Class A (1.0.0.0 to 127.255.255.255): For large networks.
- Class B (128.0.0.0 to 191.255.255.255): For medium-sized networks.
- Class C (192.0.0.0 to 223.255.255.255): For small networks.
- Class D (224.0.0.0 to 239.255.255.255): Used for multicast.
- Class E (240.0.0.0 to 255.255.255.255): Reserved for experimental use.
2.5.2 Classless Addressing
- Classless Inter-Domain Routing (CIDR) replaced classful addressing to provide more flexible and efficient IP address allocation. CIDR notation uses a format like 192.168.1.0/24, where the /24 represents the subnet mask, specifying how much of the address is network vs. host.