Mastering Network Fundamentals: OSI Model, TCP/IP Layers & Data Encapsulation

Network Fundamentals

1.1 What is a network

Network: a computer network connects computers via cables or other media to enable communication.

Communication: interaction between people, devices, or objects through a medium.

1.2 Network classifications

Local Area Network (LAN): covers a small area such as an enterprise or campus.

Metropolitan Area Network (MAN): spans a city, e.g., between Beijing and Nanjing.

Wide Area Network (WAN): covers countries or global scale, e.g., the Internet.

OSI Seven-Layer Model and TCP/IP

2.1 Reasons for layering

Simplify complex network environments for easier troubleshooting.

Standardize physical layer interfaces and hardware.

2.2 Understanding the OSI model and TCP/IP

2.2.1 Reference diagram

2.2.2 OSI layer descriptions

Application layer: human‑machine interaction interface.

Presentation layer: translates human language to binary, handling translation, encryption, compression.

Session layer: establishes, manages, and terminates sessions.

Transport layer: defines data transport mechanisms.

Network layer: selects the optimal path for data.

Data link layer: verifies MAC addresses and checks data integrity.

Physical layer: transmits bit streams as electrical signals.

Common protocols and TCP ports

Application layer:

HTTP – default TCP port 80.

FTP – file transfer, typically ports 20/21.

TFTP – simple file transfer, TCP port 69.

Transport layer:

TCP – reliable but slower.

UDP – faster but unreliable.

Network layer protocols:

IP protocol suite includes ARP, RARP, ICMP, IGMP.

Data Encapsulation and Decapsulation

3.1 Data encapsulation

Application layer: original data.

Transport layer: adds TCP/UDP header and port numbers, forming a segment.

Network layer: adds IP header, forming a packet.

Data link layer: adds MAC address header and checksum trailer, forming a frame.

Physical layer: converts the frame into electrical signals.

3.2 Data decapsulation