Understanding Network Fundamentals: OSI Model, TCP/IP, and Data Encapsulation

Network Fundamentals

1.1 What is a Network

A computer network connects computers via cables or other media to enable communication between them.

Communication refers to the exchange of information between people, devices, or objects through some medium.

1.2 Types of Networks

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

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

Wide Area Network (WAN): covers national 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 hardware interfaces at the physical layer.

2.2 Understanding the OSI Model and TCP/IP

OSI Model Layer Descriptions

Application layer: human‑machine interaction interface.

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

Session layer: establishes, manages, and terminates sessions.

Transport layer: defines data transport mechanisms.

Network layer: selects the optimal path for data transmission.

Data link layer: verifies data integrity using MAC addresses and checksums.

Physical layer: transmits bit streams as electrical signals.

Common Protocols and TCP Port Numbers

Application layer:

HTTP (Hypertext Transfer Protocol): default TCP port 80.

FTP (File Transfer Protocol): typically TCP ports 20/21.

TFTP (Trivial File Transfer Protocol): TCP port 69.

Transport layer:

TCP: reliable but slower.

UDP: unreliable and faster.

Network layer:

IP protocol: includes ARP (Address Resolution Protocol).

RARP, ICMP (Internet Control Message Protocol), IGMP.

Data Encapsulation and Decapsulation

3.1 Data Encapsulation

Application layer: original data.

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

Network layer: adds IP address 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

Reference: https://www.cnblogs.com/chenjw0608/p/17758126.html