Code-Memo

OSI and TCP/IP models

OSI model (7 layers)

1. Physical: bits, cables, radios
2. Data link: frames, MAC, switching (Ethernet)
3. Network: IP routing (IPv4/IPv6)
4. Transport: TCP/UDP, ports
5. Session: session management (often merged into app stacks)
6. Presentation: encoding, TLS, serialization
7. Application: HTTP, DNS, SSH, SMTP

TCP/IP model (common practical view)

• Link: Ethernet/Wi‑Fi
• Internet: IP
• Transport: TCP/UDP
• Application: HTTP/DNS/SSH/…

Quick mappings

• OSI L3 ≈ IP routing
• OSI L4 ≈ TCP/UDP ports
• “It’s not pinging” → L3/L2
• “DNS fails” → application layer (but depends on L3/L4)

Troubleshooting approach

• Start from the bottom (link/IP) and move up (DNS/HTTP)

Related

• IP addressing and subnetting
• Ports and sockets