OSI Model คืออะไร มีกี่ชั้น และแต่ละชั้นทำหน้าที่อะไร — คู่มือ OSI 7 Layers

OSI Model 7 Layers

OSI Model Open Systems Interconnection 7 Layers Physical Data Link Network Transport Session Presentation Application Protocol TCP/IP ISO Reference Model

Layer	ชื่อ	หน้าที่	Protocol/Device
7	Application	User Interface	HTTP, FTP, DNS, SMTP
6	Presentation	Encryption, Format	SSL/TLS, JPEG, ASCII
5	Session	Connection Management	NetBIOS, RPC, PPTP
4	Transport	Reliable Delivery	TCP, UDP
3	Network	Routing, IP Address	IP, ICMP, OSPF, BGP
2	Data Link	MAC Address, Framing	Ethernet, Wi-Fi, ARP
1	Physical	Bits, Signals	Cable, Hub, Fiber

แต่ละ Layer ทำอะไร

# osi_layers.py — OSI Model Details
from dataclasses import dataclass, field
from typing import List

@dataclass
class OSILayer:
 number: int
 name: str
 thai_name: str
 function: str
 data_unit: str
 protocols: List[str]
 devices: List[str]
 security: List[str]

layers = [
 OSILayer(7, "Application", "ชั้นแอปพลิเคชัน",
 "ให้บริการ Network แก่ User Applications",
 "Data",
 ["HTTP/HTTPS", "FTP/SFTP", "DNS", "SMTP/IMAP", "SSH", "SNMP"],
 ["Firewall (L7)", "Load Balancer (L7)", "WAF"],
 ["Input Validation", "Authentication", "WAF Rules"]),
 OSILayer(6, "Presentation", "ชั้นนำเสนอ",
 "แปลงรูปแบบข้อมูล เข้ารหัส บีบอัด",
 "Data",
 ["SSL/TLS", "JPEG", "GIF", "ASCII", "MPEG"],
 ["SSL Accelerator"],
 ["TLS 1.3", "Certificate Management", "Encryption"]),
 OSILayer(5, "Session", "ชั้นเซสชัน",
 "จัดการ Session เปิด/ปิด Connection",
 "Data",
 ["NetBIOS", "RPC", "PPTP", "SIP"],
 ["Gateway"],
 ["Session Timeout", "Token Management"]),
 OSILayer(4, "Transport", "ชั้นขนส่ง",
 "ส่งข้อมูลแบบ Reliable (TCP) หรือ Fast (UDP)",
 "Segment/Datagram",
 ["TCP", "UDP", "QUIC", "SCTP"],
 ["Firewall (L4)", "Load Balancer (L4)"],
 ["Port Security", "Rate Limiting", "SYN Flood Protection"]),
 OSILayer(3, "Network", "ชั้นเครือข่าย",
 "Routing หาเส้นทาง ใช้ IP Address",
 "Packet",
 ["IPv4", "IPv6", "ICMP", "OSPF", "BGP", "ARP"],
 ["Router", "L3 Switch", "Firewall"],
 ["ACL", "IPSec VPN", "Anti-spoofing"]),
 OSILayer(2, "Data Link", "ชั้นเชื่อมต่อข้อมูล",
 "จัดการ Frame ใช้ MAC Address",
 "Frame",
 ["Ethernet", "Wi-Fi (802.11)", "PPP", "VLAN"],
 ["Switch", "Bridge", "NIC"],
 ["Port Security", "802.1X", "VLAN Segmentation"]),
 OSILayer(1, "Physical", "ชั้นกายภาพ",
 "ส่ง Bits ผ่านสาย สัญญาณไฟฟ้า/แสง",
 "Bits",
 ["Ethernet (Physical)", "USB", "Fiber Optic", "Wi-Fi Radio"],
 ["Hub", "Repeater", "Cable", "Fiber"],
 ["Physical Security", "Cable Shielding"]),
]

print("=== OSI 7 Layers ===")
for layer in layers:
 print(f"\n Layer {layer.number}: {layer.name} ({layer.thai_name})")
 print(f" Function: {layer.function}")
 print(f" Data Unit: {layer.data_unit}")
 print(f" Protocols: {', '.join(layer.protocols[:4])}")
 print(f" Devices: {', '.join(layer.devices[:3])}")
 print(f" Security: {', '.join(layer.security[:2])}")

Python Network Programming

# network.py — Network Programming Examples

import socket

# === Layer 4: TCP Client/Server ===
# TCP Server
# server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
# server.bind(('0.0.0.0', 8080))
# server.listen(5)
# print("Server listening on port 8080")
#
# while True:
# client, addr = server.accept()
# print(f"Connection from {addr}")
# data = client.recv(1024)
# client.send(b"HTTP/1.1 200 OK\r\n\r\nHello!")
# client.close()

# TCP Client
# client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# client.connect(('example.com', 80))
# client.send(b"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n")
# response = client.recv(4096)
# print(response.decode())
# client.close()

# === Layer 3: ICMP Ping ===
# import subprocess
# result = subprocess.run(['ping', '-c', '4', '8.8.8.8'],
# capture_output=True, text=True)
# print(result.stdout)

# === Layer 7: HTTP Client ===
# import requests
# response = requests.get('https://api.example.com/health')
# print(f"Status: {response.status_code}")
# print(f"Headers: {dict(response.headers)}")

# Network Troubleshooting by Layer
troubleshoot = {
 "Layer 1 (Physical)": {
 "symptoms": "ไม่มี Link Light, สายหลุด",
 "checks": "ตรวจสาย, ตรวจ Port, ตรวจ NIC",
 "tools": "Cable Tester, show interface status",
 },
 "Layer 2 (Data Link)": {
 "symptoms": "MAC ไม่เจอ, VLAN ผิด",
 "checks": "show mac address-table, show vlan",
 "tools": "Wireshark (Frame), arp -a",
 },
 "Layer 3 (Network)": {
 "symptoms": "Ping ไม่ได้, Route ผิด",
 "checks": "ping, traceroute, show ip route",
 "tools": "tracert, pathping, MTR",
 },
 "Layer 4 (Transport)": {
 "symptoms": "Port ไม่เปิด, Connection Timeout",
 "checks": "telnet host port, netstat -an",
 "tools": "nmap, ss -tulpn, tcpdump",
 },
 "Layer 7 (Application)": {
 "symptoms": "HTTP Error, DNS ไม่ Resolve",
 "checks": "curl, nslookup, dig",
 "tools": "Browser DevTools, Postman, curl -v",
 },
}

print("\n=== Troubleshooting by Layer ===")
for layer, info in troubleshoot.items():
 print(f"\n [{layer}]")
 for k, v in info.items():
 print(f" {k}: {v}")

OSI กับ TCP/IP เปรียบเทียบ

# comparison.py — OSI vs TCP/IP
from dataclasses import dataclass
from typing import List

@dataclass
class ModelComparison:
 osi_layers: str
 tcpip_layer: str
 protocols: List[str]
 description: str

comparisons = [
 ModelComparison("Layer 7,6,5", "Application",
 ["HTTP", "FTP", "DNS", "SMTP", "SSH", "TLS"],
 "รวม 3 ชั้นบนเป็นชั้นเดียว จัดการ Application ทั้งหมด"),
 ModelComparison("Layer 4", "Transport",
 ["TCP", "UDP", "QUIC"],
 "เหมือนกัน ส่งข้อมูลระหว่าง Process (Port-to-Port)"),
 ModelComparison("Layer 3", "Internet",
 ["IP", "ICMP", "OSPF", "BGP"],
 "เหมือนกัน Routing หาเส้นทาง (Host-to-Host)"),
 ModelComparison("Layer 2,1", "Network Interface",
 ["Ethernet", "Wi-Fi", "ARP"],
 "รวม 2 ชั้นล่างเป็นชั้นเดียว จัดการ Physical + Data Link"),
]

print("=== OSI vs TCP/IP ===")
print(f" {'OSI Layers':<15} {'TCP/IP Layer':<20} {'Protocols'}")
for c in comparisons:
 protos = ", ".join(c.protocols[:4])
 print(f" {c.osi_layers:<15} {c.tcpip_layer:<20} {protos}")
 print(f" {c.description}")

# Encapsulation Process
encapsulation = [
 {"layer": 7, "action": "Application สร้าง Data", "added": "HTTP Headers"},
 {"layer": 4, "action": "Transport แบ่ง Segments", "added": "TCP/UDP Header (Port)"},
 {"layer": 3, "action": "Network สร้าง Packets", "added": "IP Header (IP Address)"},
 {"layer": 2, "action": "Data Link สร้าง Frames", "added": "MAC Header + FCS"},
 {"layer": 1, "action": "Physical ส่ง Bits", "added": "Electrical/Optical Signal"},
]

print(f"\n\nEncapsulation Process:")
for e in encapsulation:
 print(f" Layer {e['layer']}: {e['action']}")
 print(f" Added: {e['added']}")

เคล็ดลับ

• จำง่าย: Please Do Not Throw Sausage Pizza Away (Physical-Application)
• Troubleshoot: เริ่มจาก Layer 1 ขึ้นไป ตรวจสายก่อน
• TCP/IP: ใช้งานจริงบน Internet เป็น TCP/IP ไม่ใช่ OSI
• Wireshark: ใช้ Wireshark ดู Traffic ทุก Layer เรียนรู้ได้เร็ว
• Security: ป้องกันทุก Layer ไม่ใช่แค่ Firewall Layer 3-4

การนำความรู้ไปประยุกต์ใช้งานจริง

แหล่งเรียนรู้ที่แนะนำ ได้แก่ Official Documentation ที่อัพเดทล่าสุดเสมอ Online Course จาก Coursera Udemy edX ช่อง YouTube คุณภาพทั้งไทยและอังกฤษ และ Community อย่าง Discord Reddit Stack Overflow ที่ช่วยแลกเปลี่ยนประสบการณ์กับนักพัฒนาทั่วโลก

สรุป

OSI Model 7 Layers Physical Data Link Network Transport Session Presentation Application TCP/IP Protocol Encapsulation Troubleshooting Network Programming Security Wireshark CCNA